Look at the YVR.ca site for guidance, though I am not sure it's covered (not many fly U.S.=> U.S. via Canada). It usually depends on the airline and what time you arrive.

If you can get a BP for the connecting flight, you should be able to head for U.S. connections where you might have to go through a cursory Canadian document check before you get to U.S. preclearance. Hours of this may be (or is) limited however.