The thing is, he doesn't have to. The Eagles are 2-3 and Foles has proved he is at the least a capable backup. Vick's hamstring injury could neutralize his running ability, which makes him a less effective talent on the field. If Vick isn't fully healthy, it doesn't make sense for him to play.
Some would argue Foles should start regardless of Vick's health, but that is a different discussion. Chip Kelly has reiterated that Vick is the starter when healthy, so the decision as to who starts for now will be based on Vick's availability.
Kelly confirmed he hasn't officially rushed to a decision as to who's starting yet.
Again, we're never going to put [Mike] in harm's way and we're always going to do what's best for the team. The good thing is we still have three full days. We've got Thursday, Friday and Saturday of training to really get a full evaluation of where we are with this whole thing.
Reports from numerous outlets (Howard Eskin, ESPN, others) have already stated it's extremely unlikely Vick will play on Sunday, and that's probably not far from the truth. Barring an unexpected development, expect Nick Foles to start against the Buccaneers. Just don't expect Chip to officially announce it any time soon.