I wouldn't trade Vick for Manning, unless Harrison was part of the trade. Not to say that Manning and Harrison aren't among the best at thier positions, but basically they are so good because they work so well together. They are telepathic practically. If Harrison left Indy last year as a a FA, then he would not put up anywhere close to the same level of production. The same with Manning. He wouldn't connect as well with another team's top receiver as well as he does with Harrison.
But if I was going to trade Vick for another QB in this league, I would definitely look at Tom Brady, who IMO works well with a lot of receivers. McNabb would definitely be on the list to consider. Culpepper would also be a consideration.
There are a lot of QBs if we had would run this offense better than Vick. But trading Vick would mean we'd have to face him, and I think it would be hard for me to do that just for the fear of seeing him on the opposite side of the field.