The 2008 Vikings made the playoffs with Tarvaris Frerotte playing quarterback. The 2009 Vikings didn't get Brett Favre to make the playoffs again: they got him to win playoff games. This is it. This is what it was all supposed to be about. This is the Vikings' chance to go to their first Super Bowl since the 1976 season, and to win their first Super Bowl ever. And it starts with winning one home game against a very good Dallas team that does well in just about every aspect of the game.
Offensive things for the Vikings
Screens. The Cowboys have a good pass rush, and Brett Favre is good at screen plays. This seems like an excellent game to call a lot of passes to Chester Taylor and Adrian Peterson, forcing the issue with Dallas's linebackers and defensive backs, making them make plays.
Running the Ball. Yes, the Vikings have been most successful when throwing the ball well this season. But the Cowboys' four game winning streak came against teams that lacked the commitment or ability to run the ball. Adrian Peterson is the best running back the Cowboys will have seen this season, and the Vikings need to use him effectively.
Protect the passer. I think that if you give Brett Favre time to find a receiver, Sidney Rice, Percy Harvin, Bernard Berrian, and Visanthe Shiancoe will get open and the Vikes will move the ball through the air. In my view the Vikings are effective with long passing and short passing, so if Favre is given time, the Vikes can take whatever the Cowboys are leaving open.
Defensive things for the Vikings
Discipline. The Cowboys run a lot of draws and fake draws. They run a lot of bubble screens, and they'll try to get running backs off tackle. They have a quarterback that can move. The Vikings defense (the scheme and the players) need to make sure they don't get drawn out of position.
Tackling. The Cowboys will get the ball to their playmakers in space: it is critical that the Vikings make open field tackles. Make the Cowboys sustain drives and convert first downs: don't give up big plays due to poor tackling.
Play well in the red zone. The Cowboys were 2nd in the league in offensive yards (ranking 6th in passing yards and 7th in rushing yards), yet they ranked only 14th in points scored. 2nd to 14th seems like a big disconnect: those numbers suggest the Cowboys are excellent and moving the ball but merely average at scoring points.
Cover the pass coverage weaknesses. The Vikes' vulnerability against the pass are the safeties and middle linebacker--the Vikes need to scheme to protect them.
Some other things
The Cowboy and Viking defense were tied for 26th in the league with only 11 interceptions. Brett Favre threw just seven interceptions all year, Tony Romo just nine. Both Favre and Romo have been known to throw interceptions, though, so the trends could break. Which way the trend breaks could determine the outcome.
If it comes down to a final kick, the Vikings have reliable Ryan Longwell, and the Cowboys have the less than reliable Shaun Suisham. Suisham could very well make a game winner if it comes down to one kick. But his lack of reliability also might lead the Cowboys into riskier options, like going for it on fourth down or pressing for a touchdown when a field goal could be got.
The Viking pass rush has been superb at home. Last week, the Cowboys were at home and still managed to commit stupid penalties, let the play clock wind down, etc. The Thunderdome noise can make a difference.