Football Outsiders is known for doing things a little differently than a lot of other football sites on the internet. The folks at Football Outsiders analyze every single play of every single game to put together their rankings on offense, defense, and special teams to come up with something called DVOA, or Defense-adjusted Value Over Average. In order to account for it being this early in the season, they also have something called DAVE, or DVOA Adjusted for Variation Early, which takes pre-season performance into account as well.
Got all that? Okay, let's move along.
Football Outsiders currently has the Vikings at 19th in their DVOA rankings, and 18th in their DAVE rankings, which means that they think the Vikings are right around average, despite their showing on Sunday. Right now, they have the Vikings at 26th in Offensive DVOA (apparently there were six offenses in the NFL this past Sunday that were worse), 18th in Defensive DVOA, and 3rd in Special Teams DVOA.
If you're curious as to how these methods work, the folks at Football Outsiders explain all of it right here.