DirectoryInfo filePathDirectory = new DirectoryInfo(filePath);
Process a = new Process();
a.StartInfo.FileName = @"C:WindowsSystem32lpr.exe";
a.StartInfo.Arguments = "-SServerName.Domain.net -Plp " + """ + filePathDirectory + """;
a.StartInfo.UseShellExecute = false;
a.Start();
a.WaitForExit();

Dim RPname As String
RPname = FileName.ToString
Dim a As New Process
a.StartInfo.FileName = "C:Windowssystem32lpr.exe"
a.StartInfo.Arguments = "-SServerName.Domain.net -Plp " & Chr(34) & RPname & Chr(34)
a.StartInfo.UseShellExecute = False
a.Start()
a.WaitForExit()

try
{
  DirectoryInfo filePathDirectory = new DirectoryInfo(filePath);
  Process a = new Process();
  a.StartInfo.FileName = @"C:Windowssyswow64lpr.exe";  // ADAPTED to the new path!! worked!
  // use filePathDirectory.FullName!!
  a.StartInfo.Arguments = "-SServerName.Domain.net -Plp " + """ + filePathDirectory.FullName + """;
  // or change it to - found it more readable imo
  a.StartInfo.Arguments = string.Format(
                               "-SServerName.Domain.net -Plp "{0}"",
                               filePathDirectory.FullName);
  a.StartInfo.UseShellExecute = false;
  a.Start();
  a.WaitForExit();
}
catch (Exception ex)
{
  Debug.WriteLine(ex.Message);
}