As you are on one ticket, AS should be able to print your BP for you and will definitely check your bag through onto CX. YVR normally has an airside transit on the upper level of international arrivals that means you don't have to go out through customs, just have a passport check. If you have your CX BP, either through a phone app or printed by Alaska, you should be able to use this. The only problem might be if CBSA doesn't staff that transfer area at the time.