Okay, here's a dumb question about flying on partner airlines

If not, then whichever one has a counter at the airport.

It sounds like it should be American Airlines where you check-in. Just check online about your departure airport or call AA.

N/T

24 hours prior to your flight on aa website, print or put your boarding pass on your phone, and skip the line to check in at the airport. Go directly to TSA line. If you have to check baggage, it will be the AA counter, but check in online ahead of time anyway, just in case it's a long line at the counter, to preserve your seat assignment.

When you return, you will clear Customs and Immigration in Vancouver, not in the US. So plan extra time at the Vancouver airport.

More specifically it has to do with setting up a frequent flyer account, even if you aren't a frequent flyer. If you make your reservation and check in using a frequent flyer account, it will up your boarding priority. People without frequent flyer accounts board dead last.

It's also very worthwhile to use the app if you have a smart phone. Sometimes your gate assignment will change, even after you've arrived and gone through security. This happens more often when flights are delayed. The AA app will notify you immediately of any gate changes which can save you the trouble of going to the wrong gate only to find out you have to go somewhere else.