whaleboneman is correct. I fly very frequently through Vancouver. Coming from the US, your baggage checks through. You follow US to Int’l connection signage and scan your passport to get into the International departures area. No additional security either.
On return to the US, you follow transit signs and clear USA customs / immigration in Vancouver (and clear security again due to TSA rules). Again, bags check through. If you connect to a domestic Canadian flight, you go through a different transit hall with dedicated Canada immigration / customs separate from the main immigration / customs. MUCH faster. As for security, you bypass it depending on your origination point. Come from the US / UK / EU and you skip it. No idea on Thailand but I would guess Canada doesn’t consider Thailand “high security”. This is all determined by scanning the origination boarding pass.
All this said, I live near Vancouver and won’t use this flight on my Thailand trips. I’ve had bad luck with AC on delays / cancellations, this flight (if you originate in Vancouver) carries a hefty price premium, and finally departure is near midnight with an early morning arrival in BKK. OK if you have a home to go to but a PITA if you’re going to a hotel as you can’t check in for many hours. Give me JAL through Narita: reliable, great service, cost effective, and good departure / arrival times. Of course, if you fly to Vancouver from a North American city that doesn’t itself have nonstops to other Asian cities, this makes the trip one connection vs. two.