Who really knows, but neither Vick nor McNabb ever struck me as genuine leaders. Neither one seemed to really care enough about winning. Same applies to a guy like Jay Cutler or Eli Manning. It seems to me that the best leaders who who end up playing the best in big-time situations have a major aversion to losing. Brady, Manning (apart from his play in the playoffs ), Elway, Urlacher, Ray Lewis, Reggie White, etc.
I'm not sure if Aaron Rodgers has that personality characteristic or not, though. He might be an exception to the rule, being a really laid back, reasonably carefree guy who can still be a great leader and winner in clutch situations.
. . .just idle speculation. . .
Rodgers is a good QB, but Favre will always be the leader of GB. Rodgers is no Brady or Bree's or Peyton when he was playing as in terms of true leadership in this era of the Nfl.
And you're right about Ray Lewis being the Raven's leader, even though he's not the QB