Let me get this straight, Yvr to SFO is International and I always use the Maple Leaf Lounge in YVR, however when I have continued on to another destination in the USA let's say BOS that I could use the United lounge in SFO. I have always been under the impression that you must be departing on an International flight which SFO/BOS is not it is a domestic flight. Have I been wrong on this?
Yes, you are wrong. You get access as long as the flight you are departing on is part of a same-day international (including Canada) itinerary. I fly YOW-IAD-XXX and YOW-ORD-XXX (where XXX are U.S. cities) all the time and have never had a problem getting into the IAD and ORD RCCs. Similarly on XXX-IAD/ORD-YOW, you can use the RCC in both XXX and IAD/ORD.