IMO, Zimmer set a passive tone for the team by punting on 4th & 5 on the Bears 39, and that's how the Vikings played for the rest of the game. Even defensive-oriented teams have to score points to win games. When an offense struggles to sustain drives, and that offense finally strings some plays together and moves well into the opponent's side of the field, they need to go for points. Furthermore, by punting deep in the opponents territory, the defense often seems to come out with a false sense of "security" because they are protecting a long field, and the next thing you know, the opponent is down on the other end of the field looking to score.
Go for it there...if you don't make, giving the Bears the ball at their 39 isn't that big of a deal.
I understand the field position strategy, and it seemed to be almost foolproof 20 years ago, but in today's game where offenses are more prolific, IMO it seems to be rather questionable when an offense is that deep in the opponents territory.