I'd like to know this as well. From the UAD-1/Vegas users who USED to post here, it appears that Vegas 5 was the last version that worked with any success (there were rendering problems with 6 that were never fixed - don't know about 7). Its my impression from other forums that I visit that the dedictated UAD-1 users have moved on to othe apps, and rightly so. Since even the most recent UAD plugs seem to work fine in Acid 6, it really is frustrating that Sony can't or won't get it together for Vegas. I've been wanting to purchase these plugs for over 3 years but my addiction to Vegas's editing keeps me in this "well maybe next version it will work" mentality. I've been closely watching another app's development but the editing isn't quite up to par with Vegas yet. At some point it will be and I'll finally feel confident to get out of this ongoing frustrating. In the meantime, I'm working with Vegas 6 (purchased 7 but haven't even installed it yet), and continue to use outboard efx as they sound better than most of the plugs I have including most of Waves stuff. Its a tedious way to work though . . .
I use them with Vegas 5. I have not had success with V6 or V7. I have not upgraded Vegas since V5 because of this.
Since Vegas5 is DX only and UA stopped DX support, I have not used any of their newer plugs, although my initial experiments with DirectiXer seem to indicate that the DirectiXer wrapped UAD VST versions work as well as (and sometimes better than) the UAD DX versions.
Recently, I considered buying another UAD card for more plug power. But, PCI has a limited lifespan. Instead, I just purchased more outboard EQ and compression. It cost's a bit more, but it will be useful virtually forever. That said, the UAD plugs are great and a great value. I use them on every production I do.