There is a connection wizard here -
http://www.yvr.ca/en/navigating-yvr/...rough-yvr.aspx
You should be able to avoid Customs and just have a cursory immigration check. Your bags should be checked through.
Note that due to a quirk with Air Canada online check-in you can't check in until 24 hours before your AC flight (not your UA flight). So you'll be able to check in at 12:11 PM.