The problem is that he is different from normal QBs. So different that a coordinator is going to have to design a whole new system to take advantage of his talents.
How many coordinators would be willing to do that? How many head coaches have their systems in place and think, I'll be danged if we're going to scrap the system we have been working to build for one guy. It doesn't even have to be that strong a feeling, it just becomes awfully easy to weigh on the one side upheaving everything to bring in Vince Young, vs. on the other side some prospect you may have already been looking at that allows you to keep the status quo.
Not to mention all the teams that feel they HAVE a QB.
Remember what happened to Rodgers last year? Basically because he got snubbed by the 49ers, Dolphins, Browns, and Bucs, he spiraled all the way to #24 because nobody else had thought seriously or prepared themselves for the eventuality that they could draft a QB in the first round. It takes a special commitment to draft a first round QB, even MORE of a specialized commitment to take a Vince Young. Think about it...at this time last year, had anyone between the #10 and #20 area seriously considered and therefore evaluated Aaron Rodgers? No, they all took it for granted that Rodgers was top 5. Taking a QB in the first round was some OTHER team's problem to consider, not theirs.
Similarly, how seriously are the Phins or any team above us evaluating Vince Young at the moment? Don't they all just assume he goes #1 to the Texans, #2 to the Saints, or #3 to the Titans? They probably assume #4 to the Jets too.
All it would take for Vince to be available #16 might be a snub from the Titans in favor of Cutler.