Here's what I can tell you
Advantages of Visual Basic:
- Fairly easy to pickup
- Won't stress you out during school as it's not too hard to learn
Advantages of C++:
- Better language in general
- Usually can create better programs
A major, major disadvantage for C++ is its complexity. Although you feel as if you may be able to pick it up during the holidays, you'll probably be taking shortcuts to try learn it quicker, which is ALWAYS a bad idea. It's not a subject you can pickup during the holidays really, plus it puts you under the pump as you need to use your time wisely to cater for other subjects.
VB.net may teach you bad syntax habits and such, but it's a good language to learn nonetheless. Even if you were to have a career developing C++, C# or Java programs, you'll most likely end up learning Visual Basic one way or another, whether it's at University. general interest or job opportunity.
My opinion: Do VB