If you write it in VB, you can still use it in VBA, except the forms won't transfer properly. You can write it in VBA, then export the modules and forms, then import them into your VB application without any problems.
I would think that if you want to dock it, an externally defined program (VB) utilizing a dockable window for AutoCAD would be the best bet, unless you want to create a dockable container for AutoCAD .. if that is the case you better bone up on your C++ and ARX