This shouldn't be any issue but a couple things to keep in mind:
- You must start the trip in LAS! Showing up at SFO will not work.
- You are buying a ticket LAS-PVG-LAS. So in case of flight cancellations, UA is not required to route you through SFO at all. If UA only operates PVG-SFO, this should be ok. But if there is a PVG-LAX, there is some risk that you could be rerouted on that and your plan would not work so well.