More and faster cores better performance of Vegas. You will benefit from 6 cores compared to 4. Probably this is still true in VP11 that is GPU enalbed, but many of us have still this feature disabled, for other reasons...
That's a valid question, but it's also very subjective. To a degree you can have one or the other, based on your budget. The bigger your budget, the faster perfromance you can purchase.
I've been plugging away at this since single core CPUs, so I am impressed with the progess that a quad will provide. IMO, get the best CPU that your budget will allow.