Sub Button1_Click()

Dim orgFilename As String
Dim temp As String
Dim strarray(3) As String
Dim vert(4) As String
Dim vert2(3) As String
Dim newFilename As String
Dim numRows As Integer
Dim i As Integer
Dim j As Integer
Dim k As Integer
Dim segCount As Integer
Dim vertex(3, 100) As Double

Dim oldwb As Workbook
Dim newwb As Workbook

orgFilename = Application.GetOpenFilename(FileFilter:="All files (*.), *.", Title:="Please select a file")
If orgFilename = "False" Then Exit Sub
Workbooks.OpenText Filename:=orgFilename, _
            Origin:=950, StartRow:=1, DataType:=xlDelimited, TextQualifier:= _
            xlDoubleQuote, ConsecutiveDelimiter:=True, Tab:=True, Semicolon:=False, _
            Comma:=False, Space:=True, Other:=False, FieldInfo:=Array(Array(1, 1), _
            Array(2, 1), Array(3, 1)), TrailingMinusNumbers:=True
Set oldwb = ActiveWorkbook
Set newwb = Workbooks.Add

oldwb.Activate
Cells(5, 1).Select
numRows = Cells(5, 1).End(xlDown).Row

' Parse through data
segCount = 0
j = 1
For i = 5 To numRows
    If Cells(i, 1) <> "VRTX" And segCount <> 0 Then
        For k = 1 To segCount - 1
            newwb.Worksheets("Sheet1").Cells(j, 1) = "GLINE"
            With newwb.Worksheets("Sheet1")
                .Cells(j, 2) = vertex(1, k)
                .Cells(j, 3) = vertex(3, k)
                .Cells(j, 4) = vertex(2, k)
                .Cells(j, 5) = vertex(1, k + 1)
                .Cells(j, 6) = vertex(3, k + 1)
                .Cells(j, 7) = vertex(2, k + 1)
            End With
            j = j + 1
        Next k
        segCount = 0
    ElseIf Cells(i, 1) = "VRTX" Then
        ' Save vertices to save an endpoint
        vertex(1, segCount + 1) = Cells(i, 3)
        vertex(2, segCount + 1) = Cells(i, 4)
        vertex(3, segCount + 1) = Cells(i, 5)
        segCount = segCount + 1
    End If
Next i

' Save as a new file
temp = Mid$(orgFilename, InStrRev(orgFilename, "") + 1)
temp = Replace$(temp, ".pl", ".csv")
strarray(1) = Left(orgFilename, InStrRev(orgFilename, ""))
strarray(2) = "processed_"
strarray(3) = temp
newFilename = Join(strarray, "")
newwb.SaveAs Filename:=newFilename, _
            FileFormat:=xlCSV, _
            Password:="", WriteResPassword:="", ReadOnlyRecommended:=False, _
            CreateBackup:=False
End Sub

Sub main()
  openPlFile
  readPlFile
  writeCsvFile
  saveCsvFile
End Sub

On Error GoTo error_handler

Exit Sub
error_handler:
  'code to handle the error for example:
  MsgBox "There was an error: " & Err.Description
End Sub

Workbooks.OpenText Filename:=orgFilename, _
    Origin:=950, StartRow:=1, DataType:=xlDelimited, TextQualifier:= _
    xlDoubleQuote, ConsecutiveDelimiter:=True, Tab:=True, Semicolon:=False, _
    Comma:=False, Space:=True, Other:=False, FieldInfo:=Array(Array(1, 1), _
    Array(2, 1), Array(3, 1)), TrailingMinusNumbers:=True`

Workbooks.OpenText Filename:=orgFilename, _
                   Origin:=950, _
                   StartRow:=1, _
                   DataType:=xlDelimited, _
                   TextQualifier:= xlDoubleQuote, _
                   ConsecutiveDelimiter:=True, _
                   Tab:=True, _
                   Semicolon:=False, _
                   Comma:=False, _
                   Space:=True, _
                   Other:=False, _
                   FieldInfo:=Array(Array(1, 1), Array(2, 1), Array(3, 1)), _
                   TrailingMinusNumbers:=True`

Option Explicit
Sub Button1_Click()
    ' Constants are declared at the beginning of the routine.
    Const ROW_SKIP As Long = 5

    ' Avoid Dim blocks like these. It is always best to declare variables as close to their initial use
    ' as possible. This makes your code easier to read/maintain as well.
    'Dim orgFilename As String
    'Dim temp As String
    'Dim strarray(3) As String
    'Dim vert(4) As String
    'Dim vert2(3) As String
    'Dim newFilename As String
    'Dim numRows As Integer
    'Dim i As Integer
    'Dim j As Integer
    'Dim k As Integer
    'Dim segCount As Integer
    'Dim vertex(3, 100) As Double
    '
    'Dim oldwb As Workbook
    'Dim newwb As Workbook

    ' I will declare the variable name, but I will also use a name that is slightly more descriptive.
    ' This will allow others to understand what I am doing. I also encapsulate this in a function to allow for
    ' easy error handling.
    'orgFilename = Application.GetOpenFilename(FileFilter:="All files (*.), *.", Title:="Please select a file")



    ' Instead of just exiting the sub, handle this error.
    ' If orgFilename = "False" Then Exit Sub

    Dim InputFileName As String
    InputFileName = GetInputFileName

    If InputFileName = vbNullString Then
        ' We can add a messagebox here if needed. For now, we just exit the routine silently.
        Exit Sub
    End If

    ' For your field info here, you are using an uninitialized, undeclared, array. What effect are you intending to achieve?
    Workbooks.OpenText _
                Filename:=orgFilename, _
                Origin:=950, _
                StartRow:=1, _
                DataType:=xlDelimited, _
                TextQualifier:= _
                xlDoubleQuote, _
                ConsecutiveDelimiter:=True, _
                Tab:=True, Semicolon:=False, _
                Comma:=False, _
                Space:=True, _
                Other:=False, _
                FieldInfo:=Array(Array(1, 1), Array(2, 1), Array(3, 1)), _
                TrailingMinusNumbers:=True

    ' I declare more descriptive workbook variable names, and separate the assignments.
    ' Set oldwb = ActiveWorkbook
    ' Set newwb = Workbooks.Add

    ' I am changing this to a Sheet reference since you seem to be referring to the ActiveSheet implicitly, and not just the ActiveWorkbook
    Dim CurrentWorksheet As Worksheet
    Set CurrentWorksheet = ActiveSheet

    ' While the default scope of `Workbooks.Add` is `Application.Workbooks.Add` it is better to be explicit.
    Dim OutputWorkbook As Workbook
    Set OutputWorkbook = Application.Workbooks.Add

    ' No need for Activate. Try to avoid this behavior.
    ' oldwb.Activate

    ' Avoid Select as well
    ' Cells(5, 1).Select
    ' numRows = Cells(5, 1).End(xlDown).Row

    ' Declare new variable, and qualify the range reference when finding the row. Without the qualifying reference
    ' to `CurrentWorkbook` the `Cells` reference refers to the `ActiveWorkbook`.
    Dim NumberOfRows As Long
    NumberOfRows = CurrentWorksheet.Cells(5, 1).End(xlDown).Row

    ' Instead of making changed within the loop, I am just going to rewrite it to make changes easier to read.
    ' Parse through data
    'segCount = 0
    'j = 1
    'For i = 5 To numRows
    '    If Cells(i, 1) <> "VRTX" And segCount <> 0 Then
    '        For k = 1 To segCount - 1
    '            newwb.Worksheets("Sheet1").Cells(j, 1) = "GLINE"
    '            With newwb.Worksheets("Sheet1")
    '                .Cells(j, 2) = vertex(1, k)
    '                .Cells(j, 3) = vertex(3, k)
    '                .Cells(j, 4) = vertex(2, k)
    '                .Cells(j, 5) = vertex(1, k + 1)
    '                .Cells(j, 6) = vertex(3, k + 1)
    '                .Cells(j, 7) = vertex(2, k + 1)
    '            End With
    '            j = j + 1
    '        Next k
    '        segCount = 0
    '    ElseIf Cells(i, 1) = "VRTX" Then
    '        ' Save vertices to save an endpoint
    '        vertex(1, segCount + 1) = Cells(i, 3)
    '        vertex(2, segCount + 1) = Cells(i, 4)
    '        vertex(3, segCount + 1) = Cells(i, 5)
    '        segCount = segCount + 1
    '    End If
    'Next i

    ' Assumes that the UsedRange of the Input sheet is the data we need
    Dim InputData As Variant
    InputData = CurrentWorksheet.UsedRange.Value

    Dim SegmentCount As Long

    Dim i As Long
    Dim j As Long
    Dim k As Long

    j = 1

    ' Re-creating your vertex array though it is not at all clear what it is being used for.
    Dim Vertices As Variant
    ReDim Vertices(3, 100)

    ' I use a constant variable instead of 5 here since the 5 may change, and it can be difficult to track it down later.
    For i = ROW_SKIP To NumberOfRows
            ' Note: This will always return false on the first pass since SegmentCount will always equal 0
            If InputData(i, 1) <> "VRTX" And SegmentCount <> 0 Then
                For k = 1 To segCount - 1
                    OutputWorkbook.Worksheets("Sheet1").Cells(j, 1) = "GLINE"
                    With OutputWorkbook.Worksheets("Sheet1")
                        .Cells(j, 2) = Vertices(1, k)
                        .Cells(j, 3) = Vertices(3, k)
                        .Cells(j, 4) = Vertices(2, k)
                        .Cells(j, 5) = Vertices(1, k + 1)
                        .Cells(j, 6) = Vertices(3, k + 1)
                        .Cells(j, 7) = Vertices(2, k + 1)
                    End With
                    j = j + 1
                Next k
            SegmentCount = 0
        ElseIf InputData(i, 1) = "VRTX" Then
            Vertices(1, SegmentCount + 1) = InputData(i, 3)
            Vertices(2, SegmentCount + 1) = InputData(i, 4)
            Vertices(3, SegmentCount + 1) = InputData(i, 5)

            SegmentCount = SegmentCount + 1
        End If
    Next i

    ' This can be condensed into a much more concise format

    ' Save as a new file
    ' temp = Mid$(orgFilename, InStrRev(orgFilename, "") + 1)
    ' temp = Replace$(temp, ".pl", ".csv")


    ' strarray(1) = Left(orgFilename, InStrRev(orgFilename, ""))
    ' strarray(2) = "processed_"
    ' strarray(3) = temp

    ' newFilename = Join(strarray, "")

    Dim OutputFileName As String

    ' This takes care of the entire operation in one line, and allows others to see what these operations are being used for.
    OutputFileName = Left(orgFilename, InStrRev(orgFilename, "")) & "processed_" & Replace$(Mid$(orgFilename, InStrRev(orgFilename, "") + 1), ".pl", ".csv")

    OutputWorkbook.SaveAs Filename:=OutputFileName, _
                    FileFormat:=xlCSV, _
                    Password:="", _
                    WriteResPassword:="", _
                    ReadOnlyRecommended:=False, _
                    CreateBackup:=False
End Sub
Private Function GetInputFileName() As String
    ' I use a variant declaration because the return of `Cancel` is the Boolean false.
    Dim InputFileNameResult As Variant

    InputFileNameResult = Application.GetOpenFilename(FileFilter:="All files (*.), *.", Title:="Please select a file")

    If Not InputFileNameResult Then
        GetInputFileName = InputFileNameResult
    Else
        ' You can handle this as needed. For now, we just assume the user wants to exit the routine.
        ' As such, we do nothing.
    End If
End Function

Option Explicit
Sub Button1_Click()
    ' Constants are declared at the beginning of the routine.
    Const ROW_SKIP As Long = 5

    Dim InputFileName As String
    InputFileName = GetInputFileName

    If InputFileName = vbNullString Then
        ' We can add a messagebox here if needed. For now, we just exit the routine silently.
        Exit Sub
    End If

    ' For your field info here, you are using an uninitialized, undeclared, array. What effect are you intending to achieve?
    Workbooks.OpenText _
                Filename:=orgFilename, _
                Origin:=950, _
                StartRow:=1, _
                DataType:=xlDelimited, _
                TextQualifier:= _
                xlDoubleQuote, _
                ConsecutiveDelimiter:=True, _
                Tab:=True, Semicolon:=False, _
                Comma:=False, _
                Space:=True, _
                Other:=False, _
                FieldInfo:=Array(Array(1, 1), Array(2, 1), Array(3, 1)), _
                TrailingMinusNumbers:=True

    ' I am changing this to a Sheet reference since you seem to be referring to the ActiveSheet implicitly, and not just the ActiveWorkbook
    Dim CurrentWorksheet As Worksheet
    Set CurrentWorksheet = ActiveSheet

    ' While the default scope of `Workbooks.Add` is `Application.Workbooks.Add` it is better to be explicit.
    Dim OutputWorkbook As Workbook
    Set OutputWorkbook = Application.Workbooks.Add

    ' Declare new variable, and qualify the range reference when finding the row. Without the qualifying reference
    ' to `CurrentWorkbook` the `Cells` reference refers to the `ActiveWorkbook`.
    Dim NumberOfRows As Long
    NumberOfRows = CurrentWorksheet.Cells(5, 1).End(xlDown).Row

    ' Assumes that the UsedRange of the Input sheet is the data we need
    Dim InputData As Variant
    InputData = CurrentWorksheet.UsedRange.Value

    Dim SegmentCount As Long

    Dim i As Long
    Dim j As Long
    Dim k As Long

    j = 1

    ' Re-creating your vertex array though it is not at all clear what it is being used for.
    Dim Vertices As Variant
    ReDim Vertices(3, 100)

    ' I use a constant variable instead of 5 here since the 5 may change, and it can be difficult to track it down later.
    For i = ROW_SKIP To NumberOfRows
            ' Note: This will always return false on the first pass since SegmentCount will always equal 0
            If InputData(i, 1) <> "VRTX" And SegmentCount <> 0 Then
                For k = 1 To segCount - 1
                    OutputWorkbook.Worksheets("Sheet1").Cells(j, 1) = "GLINE"
                    With OutputWorkbook.Worksheets("Sheet1")
                        .Cells(j, 2) = Vertices(1, k)
                        .Cells(j, 3) = Vertices(3, k)
                        .Cells(j, 4) = Vertices(2, k)
                        .Cells(j, 5) = Vertices(1, k + 1)
                        .Cells(j, 6) = Vertices(3, k + 1)
                        .Cells(j, 7) = Vertices(2, k + 1)
                    End With
                    j = j + 1
                Next k
            SegmentCount = 0
        ElseIf InputData(i, 1) = "VRTX" Then
            Vertices(1, SegmentCount + 1) = InputData(i, 3)
            Vertices(2, SegmentCount + 1) = InputData(i, 4)
            Vertices(3, SegmentCount + 1) = InputData(i, 5)

            SegmentCount = SegmentCount + 1
        End If
    Next i

    ' This takes care of the entire operation in one line, and allows others to see what these operations are being used for.
    Dim OutputFileName As String
    OutputFileName = Left(orgFilename, InStrRev(orgFilename, "")) & "processed_" & Replace$(Mid$(orgFilename, InStrRev(orgFilename, "") + 1), ".pl", ".csv")

    OutputWorkbook.SaveAs Filename:=OutputFileName, _
                    FileFormat:=xlCSV, _
                    Password:="", _
                    WriteResPassword:="", _
                    ReadOnlyRecommended:=False, _
                    CreateBackup:=False
End Sub
Private Function GetInputFileName() As String
    ' I use a variant declaration because the return of `Cancel` is the Boolean false.
    Dim InputFileNameResult As Variant

    InputFileNameResult = Application.GetOpenFilename(FileFilter:="All files (*.), *.", Title:="Please select a file")

    If Not InputFileNameResult Then
        GetInputFileName = InputFileNameResult
    Else
        ' You can handle this as needed. For now, we just assume the user wants to exit the routine.
        ' As such, we do nothing.
    End If
End Function

SomeWorkbook.Activate
Sheets("SomeSheet").Select
msgbox Cells(1,10)

msgbox SomeWorkbook.Sheets("SomeSheet").Cells(1,10).Value