It will work as far as China is concerned. For TWOV purposes, they look at the itinerary as being YVR-PVG-SFO (Canada-China-USA). They care only about the actual plane that brings you to China and the one that takes you out of China. Where you were before and where you'll be after don't matter. The Wiki in the very long relevant thread in the FT China forum goes into great detail about this.
The problem identified in this not-as-long thread are gate agents and check-in agents outside China who don't understand how this works. Admittedly, it does sound odd, but China is perfectly willing to allow this.