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Enrollment on Arrival happens at US immigration when you're entering the US. As such, you would not do it when you arrive into Vancouver (you'll be entering Canada and will not see US officials) or when you land back in SFO (you'll arrive as a domestic flight since you already went through US immigration preclearance in Canada).
Vancouver is listed as a participating Enrollment on Arrival airport, so you can complete your enrollment there, when you go through US immigration to board your return flight back to the US.
According to that page, it is found at US immigration booths 21 and 23 at Vancouver and open from 6am to 6pm.