Is it possible to have it so that VAssistX is disabled by default on startup of VS2010? I don't use it very often, but I do want to be able to turn it on when I need to.
Unfortunately VA is not designed to load disabled. You could try using the dialog:
IDE tools menu -> Extension Manager...
and disabling VA, but enabling VA will require an IDE restart.
Alternatively you could use VA's options dialog, and set everything to "Min" on the Quick Config page. This should have the desired effect, and you can then quickly enable VA by using the Quick Config page as and when you want to.
Is this something that could be implemented in a future release? I'm primarily a C# developer, with the need to use C++/CLI once in a while. Only then do I really need the benefits of VAssistX.