Mind games is one thing, but do you really believe the Vikings fell for it? They wasted no time looking at and studying Rush all week.
The only way that the mind games might work, is if the Vikings coaching staff and players really, truly believed that Dak wasn't going to play and prepared more for Cooper Rush or went into the game with the mindset that they were going to pick up an easy win with Cooper Rush as the Cowboys QB. But, then Dak actually ends up playing and has a huge game against the Vikings defense.
The Vikings will not be caught off guard with Cooper Rush, as I don't see them having any trouble stopping and limiting him at all.