Its showing F4 Y0 right now. Since its a domestic 767 flying ORD-SFO and with 210 seats in economy the odds are very good that there are at least 4 pax on the UG waitlist that will fill the remaining F seats, so there's almost no chance of an op-up.

However, a VDB is a real possibility.

My strategy would depend on my seat assignment (or lack thereof). If I had a decent seat assignment I'd wait until the airport to check in. Then I'd ask at the earliest opportunity to be placed on the volunteer list. Get to the gate about 55 minutes prior to departure and ask the GA about the need for VDBs.

If I didn't have a decent seat assignment I'd OLCI immediately and see if I could get one. Then I'd take the same VDB steps above, regardless of the Y seat.

Right now the 5:05 flight UA907 is showing F9 Y1 so if they offer you that flight you might have a better shot at scoring a free F seat.