ehhhh, the way i see it is that they were both replacements for an amazing band that had obviously reached its peak by the 1990's. When i think grateful dead, and i know im not alone, bruce and vince never cross my mind. The dead knew what they wanted and didnt want, and a new "face" after brent died was not what they wanted.
As a keyboardist and serious Brent fan, Vince was never my favorite player by any means. To say he sucked is a bit much though, IMHO. True, he may not have played sounds/solos the way I'da liked him to, or the way people were accustomed to, but, that doesn't mean he sucked. Most people don't realize that Vince had a pretty heavy hand in getting them to change things up (All to Much, Baba O'Reily) and break out some of the older stuff (Here Comes Sunshine for ex.)
If he was up to high in the mix, or his vocals were overpowering,I really don't think that was his doing. They did have a sound man, right. As for stuff that might be worth a listen, check out some 93 shows with him. Actually some decent stuff if you caught the right show.
The man did have some very serious issues with chronic and severe depression and in fact did try to off himself on Ratdog's bus one tour. That was basically the end of Vince and the GD organization's relationship. From what I read, Vince made a lot of efforts after Jerry died to get the core four back together and tried real hard to make ammends and continue to play music in some form with thoses guys. From what I read, they pretty much exiled him, which of course didn't help his depression. He was apparently devestated when they didn't want him to be a part of TOO/The Dead. Couple that with a very insane marriage and a drinking problem and you know what happened.
It is truly sad to think the guy was playing huge stadiums with 80K crowds and ended up hanging out playing small gigs with bar bands. I actually feel bad for him. I get this mental image of a wounded puppy dog trying to get someone to help him....
At least he got his 15 minutes and maybe a little more.

