Pettersson steered Brock Boeser's shot behind Robin Lehner in the Vegas goal 3.19 into the third period.
Vancouver goalkeeper Jacob Markström, who has been in the team's 14 previous playoff games, threw in the towel shortly before the game.
It has not been said whether it is an injury or illness.
But substitute Thatcher Demko played great and saved 42 shots and the victory for Vancouver, which now has 2-3 in games in the best of seven.