First off, I believe Peyton would be in contention for the 'comeback player' this year, but as far as quarterbacks go, he's not the mvp.
If you want Quarterbacks as MVP, there is no better place to look Arraon Rodgers. The fact Manning came off of injury shouldn't factor in. It's about overall stats. Rodgers has one more touchdown than Manning does, two fewer picks, and a much weaker defense standing behind him. (Compare Denver's number four defense to Green Bay's number fourteen defense.)
It's obvious which person holds more of the team on his back. Without a substantial rushing presence or defense, Green Bay has one answer to pretty much everything; Rodgers, Rodgers, and more Rodgers.
However, he's the only player in the league whom I believe could even rival Peterson in an opportunity to win MVP. Considering Minnesota has the absolute worst pass ranking in the league, and the number twenty defense, Peterson is the only viable option. People know he's coming. And yet they still can't stop him. Greatest runningback in the NFL, now and all time, IMO.
My problem mainly is their record. Peterson has pretty much accepted the role of being the QB type leader of this team. He has accepted he is the main offensive weapon. If they don't make the playoffs, the tangibles towards his offensive capabilities are limited by the fact that he can't attribute to successful, game winning schemes like a Quarterback can on the field. Because Rodgers can call audibles and keeps plays running so smoothly on the field, he has an automatic value which Peterson, as a Running Back, can't quite match up to.
If the Vikes make the playoffs, I would say Peterson has it locked. If they don't, I would lean Rodgers. The man is a leader and the best Quarterback in the NFC. (Just above Matt Ryan in that respect.)