Couple things for me:
1. If we're talking this season, there's nothing special about the Vikings D. They're extremely pedestrian in pretty much all phases. Vikings ability to win games this season has predominantly been through the output of their offense. Which would be expected, because from a personnel standpoint, they've got a lot of talent on offense.
2. Cousins track record for playing poorly in "primetime" games is well documented. I personally don't put a ton of stock into the whole "primetime games" thing, mostly because they don't count any different than a normal 1pm game, and both the public and the media tend to overreact on both sides to national games. There's no evidence they're more indicative of how a team will perform in the long run.
3. I think Cousins gets unfairly shit on by the public way too much, BUT, that being said, he's likely to be the 2nd best QB on the field in pretty much any playoff game he'll play in, and postseason play is largely predicated on the output of the QB.