As others have said, the stars will need to align, and you will have to hand-hold this in order to try to make this work.

You have to fax in your BPs, hope that Customer Care can quickly act on them, and then hope that there is space SFO-YVR.

And if any of that fails, what's your backup plan?

UA sells SFO-SEA for $178 RT on your dates. AS sells OAK-SEA for the same. I would just book that and drive up. Use your miles when you know they're in there, and for a more expensive trip.