Be very careful on arrival in YVR with the signs - you want to follow the signs to US departures, and not enter the pathway to arrivals, even if staff are trying to direct people that way. You will enter the US pre-clearance zone, redo another security check, then clear US immigration and customs in that area, and exit right into the pre-clearance departure zone. If you make a mistake, you would be dumped into the domestic arrivals with what could be a very long wait to enter Canada unless someone is willing to take you back upstairs.

Also make sure you are legally eligible to enter Canada - even if you're eligible to enter the USA, you still need to satisfy Canadian entry requirements just in case there is an issue (delay, cancellation, etc) that requires you to backtrack into Canada. ANA will be checking for this at check-in or boarding.