Order by case in dbCommand.CommandText
Protected Sub AddData()
    Dim cn As OleDbConnection = New OleDbConnection(ConfigurationManager.AppSettings("ConnectionString"))
    cn.Open()
    Dim dbCommand As New OleDbCommand
    dbCommand.Connection = cn

    dbCommand.CommandText = "SELECT * from tblPosts ORDER BY CASE PriorityLevel WHEN 'Red' then 1 when 'Orange' then 2 ELSE 3 END"

    Try
        dbCommand.ExecuteNonQuery()
        Dim objdatareader As OleDbDataReader = dbCommand.ExecuteReader()
        While objdatareader.Read()
            Dim PostID As Integer = CInt(objdatareader("ID"))
            PopulateImages(PostID, cn)
            PopulateCategories(PostID, cn)
            rssFeed.AddItem(objdatareader, ImageList, CategoryList)
            ImageList.Clear()
            CategoryList.Clear()
        End While

        objdatareader.Close()
    Catch myerror As OleDbException
        Dim strErr As String = "There was an error updating the database: " & myerror.Message
    End

declare @tblPosts table(
  PriorityLevel varchar(10)
  )

  insert into @tblPosts
  values('Orange'),('Red'),('Green'),('Orange')

  SELECT * from @tblPosts ORDER BY
  CASE PriorityLevel WHEN 'Red' then 1
                     when 'Orange' then 2
  ELSE 3 END

declare @tblPosts table(
  PriorityLevel varchar(10)
  )

  insert into @tblPosts
  values('Orange'),('Red'),('Green'),('Orange')

  SELECT * from @tblPosts ORDER BY CASE
  PriorityLevel WHEN 'Red' then 1
                when 'Orange' then 2
  ELSE 3 END PriorityLevel

SELECT tblPosts.*
FROM tblPosts
ORDER BY IIf(PriorityLevel='Red',1,IIf(PriorityLevel='Orange',2,3));

SELECT DISTINCT
    tblPosts.PriorityLevel,
    IIf(PriorityLevel='Red',1,IIf(PriorityLevel='Orange',2,3)) AS order_by
INTO PriorityLevelOrder
FROM tblPosts;

SELECT tblPosts.*
FROM
    tblPosts INNER JOIN PriorityLevelOrder
    ON tblPosts.PriorityLevel = PriorityLevelOrder.PriorityLevel
ORDER BY PriorityLevelOrder.order_by;