If you are transiting Intl -> US you 'usually' don't not have to claim your bags any more. You do have to talk to a US CBP officer at YVR.

There is a wizard here that explains the steps:

http://yvr.ca/en/navigating-yvr/pass...rough-yvr.aspx

I say 'usually' because some carriers don't interline bags. For example, Transat may not interline bags to AC. If that's the case, you may need to claim your bags at YVR and then recheck them to the USA.