I've not had any problems. I have printed discs via the Bravo II and burned discs via the Bravo II while waiting on an encode. I have also used Adobe Fireworks and Photoshop while encoding. This is with Vp8 while doing 2 pass VBR.
I don't think so. I think others have reported that this worked fine for them, but that was in previous versions. I would think that if it can handle Photoshop being up and active, it should be able to handle 6 or 10 instances of Vegas being up :)
with vegas 4 & 6 i'd have ~5 copies open at once. I did that even on my P3-667. :) Still relatively snappy, especially if you change the priority in the "processes" tab to below normal.