Yes, you will clear US immigration and Customs in Vancouver and connect as a domestic passenger in SEA. (And to be clear: it's not a choice. You have no option to clear in SEA instead of YVR.) The flip side is you need to arrive at YVR with extra time before your flight. Pre-clearance can be very fast, but I've also had it take more than an hour (including security).

US Customs can technically do further international arrivals checks at SEA, but I have never had that happen; the planes always arrive at normal AS domestic gates.