Depending on your fare construction, you might get a cheaper fare if you match the front & end of your itinerary.
(i.e. YVR-SEA-...-SEA-YVR or YVR-SFO-...-SFO-YVR) You might also consider going via NRT on the outbound as well (lots of shower suites at the UA RCC and don't have to sit for 15 hours in Y for the SFO-HKG flight)