If the Vikings avoid starting out 0-2 (at NO, vs. MIA), they should be alright.
I'm hesitant to anoint the Vikings as a real player in the NFC and that's just from past experience. Far too often, when I've seen talented teams that suffered cruel playoff losses - either crushing last-second defeats or shocking blowout where they "didn't show up. (The 2009 and 1998 Vikings are the former, the 2000 Vikings are the latter), they can keep it together for the first few weeks of the next season but at the first sign of adversity...it falls apart like a Fiat.
Recent evidence:
1998 Packers
1999 Vikings
2001 Vikings
2005 Packers (maybe that was the culmination of the 2002-03-04 losses)
2008 Packers
2008 Cowboys
1996 Chiefs
1998 Chiefs
2004 Chiefs
2001 Titans
2004 Titans
2009 Titans
2003 Raiders (they've been in a tailspin since the Super Bowl loss)
The Colts and Chargers seem to be the only teams immune to this in recent years, along with the 1997 Broncos. Even after humiliating home losses in the playoffs, they bounce right back, win their division and set themselves up for the next year's missed field goal or killer interception at home in January. No drop off for them.
As for the Vikings, Favre will be 41 but his offensive line will also be a bit older. Will he go 33 TD/7 INT again? Not as many breathers on the schedule - the Vikings could easily turn 0-2 into 2-5, that is why those games are so critical.
The second it goes sideways (and it probably will in those first 7 games at some point), will Favre be the rallying point? Or will any groundswell for a mobile quarterback take over? Favre has had a hard time putting together two great years in a row. That would be my big red flag for the Vikings.
Yes, he is their best option -- but they need to get to Halloween with a .500 mark. Not sure about that.