First of all, I highly doubt the Vikings would win 7 games with Toby Gerhart as their RB.
Second of all, as much as the "true" definition of MVP is most valuable to their team, you can't deny that stats and achievements play a big part as well. He could set a major rushing record (perhaps the top honour an RB can have), or at the very least, do something that has only been done 6 times in history.
Peterson would get my vote.