Despite starting the contest brightly, Galaxy found themselves behind in only the fifth minute.
The hosts were hit on the counter attack by Vancouver and Darren Mattocks showed the confidence to slot the ball past Josh Saunders to give his side the lead.
Galaxy had opportunities to level the scores before the break, with Robbie Keane being denied by a Brad Knighton save following a flowing move from the home side.
LA's dominance continued in the second half but they were not rewarded for their pressure until the 69th minute.
David Beckham picked out Mike Magee with a corner and the midfielder equalised in spectacular fashion with a well-placed volley from the edge of the area.
The turnaround was completed three minutes later when Juninho was fouled by Martin Bonjour inside the area, allowing Landon Donovan to rifle home the resulting penalty to keep up Galaxy's hopes of retaining their title.