No, because your final destination is in Canada, you will have to clear Canadian customs and immigration in YVR. Not very familiar with YVR, but so long as your UA flight is on time, 77 minutes is not terribly unreasonable. But for more security, you might see if you can get an earlier flight from SFO to YVR. I would probably do that so long as it doesn't push the connecting time up over 4 hours.