Are they just offering him a plea deal because a lot of rich and powerful people want this to go away as soon as possible rather than going to trial?
Two guesses:
1) A bird in the hand is better than two in the bush. A plea deal avoids the chance of getting an OJ juror or two and losing the whole thing.
2) This is the larger one: the information Vick probably can provide about the whole eastern US dog fighting racket along with its associated gun and drug crimes. As the evidence is stacking up it looks more and more like Vick was a godfather of the racket, going so far as to create a dog fighter's Disney World. If so, he knows the racket and the people in it just about as well as Al Capone knew the Chicago Mafia, and could provide enough information to break the whole thing wide open.