I dunno, I think you go ahead and you make the talent evaluation. The only young player in the secondary that can be worse than a veteran like Vincent and still make it on the roster due to potential would be Jason Allen. Other than that, if Vincent looks better than Michael Lehan at corner, I don't see why you don't make the move even with the age.
I think too many coaches are reluctant to add aged veterans that are obviously better than younger players...erring on the side of the potential. But active evaluation means figuring out just where that young player has the potential to be in a year or two and if you don't see him being a very good starter then I don't see much reason for keeping him over someone better. You have to be willing to be wrong every now and then...you have to be willing to let Marc Bulger go and not think it's the end of the world just because he ended up a pro bowl caliber player on the Rams.
That's why when teams are rebuilding I always get confused when they maintain a young roster. The important thing is to establish the patterns necessary for success, and IMO that means getting veterans that know the business, know how to establish winning patterns, and then getting some young people under them that can inherit the establishment of patterns that lead to success.