The great thing is that Vick is a system QB. The system helps Vick win if he doesn't perform lights out. But the system won't work if Vick isn't in the game. It's what Vick's presence can do for the running game.
My dad is a Steeler fan and we were discussing this very topic this past weekend. Ben Roethlisberger and Michael Vick are the QBs on the best running teams in the league (Falcons are #1 in rushing yards and attempts, Steelers are #3 in attempts, and #5 in yards). But when either player is out of the game, their offenses lose somehting.
Logically, it doesn't make sense. Since if they run the ball so well, they shouldn't need their starting QB in order to keep doing it. But for both, their very presences only enhance the running game. Rarely will either Big Ben or Vick beat you in the air. It's only going to be a few times if ever in a season where either guy is going to throw the ball 30+ times, and the offense only gain 90 or so yards on the ground and their teams will win.
But in Vick's case, he brings that threat of running himself that forces defenses to not play the run the same, nor can they play the pass the same. You don't need to use a lot of 3-WR sets when you have Vick on the field, because he alone spreads out defenses.
In Big Ben's case, with Hines Ward and the emerging Heath Miller (who WAS the steal of the draft), if you play the run too hard, the Steelers have the ability and talent to make you hurt in the passing game. 50% of hte passes thrown
(not caught, but thrown) to Hines Ward are first downs (Chad Johnson and Ernest Wilford are the only WRs that boast better numbers in that area). So the Steelers can hurt you and move the ball in the air.
So it's interesting that the two best running teams in the league need their QB probably more than any other player.