I did the same thing you did and invested in that same video card only to experience many of the problems you write about. I keep Vegas 10 on my desktop, I use Vegas 11 with all the accelerators turned off for most of my work and I haven’t had many problems.
It is very disappointing that Sony cannot get this right. GPU acceleration is used in other editing software very efficiently. I am committed to Vegas and can only hope they figure out what the problem is with the next update. I like most of us have contacted them about the problem, but they offer no real solutions.