Due to USA preclearance in YVR your flights will arrive and depart from terminal 3 in SFO, so no need to go landside and reclear security. 48 mins is lots of time, assuming your flight from YVR is remotely on time. If not, it depends how long you're delayed of course. That said, YVR has a pretty good on-time departure record. SFO not so much, but usually weather related delays there affect all flights so there's a good chance your outbound will be delayed as well.
Overall your chances are very good but anything can happen these days, especially with the 'new' United.