You could learn VB6 only if it's your first programming laungage because it is easy. Usually people start with VB6. But nowdays I think that starting with vb.net would be better as in my opinion vb6 and vb.net are almost same(well...they have got some differences but I remember that I didn't face any problems when I tried to 'move' into vb.net).
Well, vb.net can help you out with many problems as it is easy and for small programs programming would be a piece of cake but I don't think that you need it as you already know C++.
C#/C++ can be used almost everywhere. Every platform or engine usually has something that can be programmed with C#/C++. Visual basic is still alive because people usually start with it. It's not as spread as it's C++/C#