I assumed that vb.net is Microsoft Visual Basic .Net right?
Therefore it is not a linux issue.
But as I remember (I have give up windows for half a years), you will have to use stream to do so.
Go to MSDN website or get the MSDN CD and search for the details about how to use stream.
I have written a program on VB.Net which can operate with text file.
It didn't solve your problem directly, but you can use it for an example (or use it instead of notepad) about how to use stream.
That is not a good soft but I hope it have something helpful for you.
Download it at http://super-write.sourceforge.net