What happened when you nominated a different encoding in the override of the OpenTextFile method? What encodings did you try? Have you compared the file that works against the file that doesn't, and could you identify what the difference between them was? Note that this error is not usual - there is something different about the way that you are creating or using that file that is causing a problem that most people would not experience.
While I am sure it is possible to use a lot of the concepts you learned while using Python, the detail in VB .Net will be different, and it's always the detail that catches you out.