You might consider staying the night before your departure from YVR at the Vancouver Airport Fairmont. It is inside the terminal and just one level (by elevator or escalator) above the U.S. airlines checking area. It takes just 5 minutes or less to go from the hotel to the airlines checkin. It's a very nice experience as well as there are runway facing rooms so you can watch planes coming and going. They even have a booklet with aircraft photos so you can identify the planes. Not cheap, but very convenient.

See: http://www.fairmont.com/vancouver-airport-richmond/

The Skytrain is very nice and they have a station inside the airport. But it is a good walk from the train station to the checkin. Also it looks like the first trains in the morning on the Canada Line (the line that goes into the airport) is @ 5- 5:30 AM.

One other thought. I stayed at an AirBNB that was in a high rise condo right above a Skytrain station. So I could take the elevator down to the Skytrain and whizz off to the airport, or all over Vancouver. You might consider staying at a place like that until your last day and move to the Fairmont at the airport.