For someone arriving YVR on a intl flight and connecting to US the same day (immediate), you can bypass the Canadian immigration, turn to your right before you take the elevators going down to the arrival immigration hall. You will then be directed to the US immigration hall. This does not work for people from countries requiring Canadian visa to enter Canada, though. I had done it multiple times on my way to Chicago as HKG/YVR/ORD. When checking in at Hong Kong, the agent always asked me whether I plan to visit Vancouver or if I would like to bypass the Canadian immigration.

If the connection is overnight, though, AC always check the baggage to the overnight point.