The Seattle Sounders advance if they win. It's rather simple. If they lose to the Vancouver Whitecaps, or there is a goal-scoring draw then the 'Caps advance. Extra Time and possibly penalty kicks are only possible with a 0-0 draw.
I say again - win and advance, win and advance, win and advance.
Clint Dempsey returns to the MLS Cup Playoffs for the first time since 2015. He will lead an attack that scores nearly two goals a game at home against a Whitecaps defense that gives up 1.7 goals a game on the road.