::Is there a possibility to use optional parameters when defining a VB.NET function? If I add one additional parameter, wherever I have a call to that function, the compiler will immediately show the line as an error.

I think the only way you can do that is to overload your method. IOW, create another method of the same name with the parameter combination you want:

Namespace Morning
  Public Class Coffee
    Public Sub Brew()
      Console.Write("Ahhh!")
    End Sub

    Public Sub Brew(ByVal tlDecaf As Boolean)
      Console.Write("Ikkers!")
    End Sub
  End Class
End Namespace