You will clear US customs and immigration in SFO, and then have the opportunity to re-check your bags to YVR after clearing customs. In your case, you would just push your bags past the re-check area and exit the airport. The airlines consider hidden city ticketing to be a violation of their contract with you, but I don't think there will be anything that would force you to check your bags back in for a flight you don't plan to take. It wouldn't hurt to tear off the luggage tags showing YVR as a final destination once you have possession of them at SFO. If any bags get mishandled, they are going to end up in Vancouver