I feel like it is an incredibly inconsistent metric and as such has very little value. If a good qb has huge hands, you can point to that as something he uses well, but as many people have alluded to, many great qbs have had small hands. Maybe Kurt Warner would have been better with bigger hands, but thats just nonsense to consider.
Vick's hands have nothing to do with his fumbles just like his hands have nothing to do with his interceptions. He doesn't seem to have any problems gripping the ball as he is capable of throwing a beautiful ball. However, because he has small hands, people can point to that, just like the opposite would hold true of a successful qb with big hands. It really is nonsense.