Unless you are flying on a really late night flight out of YVR, you'll most certainly pass through the US CBP at YVR. Pre-clearing the US immigration in Canada is so much nice and easy, it just feels like an exit checkpoint out of Canada, they will ask a quick question or two, you'll be headed your way and your bags will be checked all the way through to your final destination! So when you make it to SEA, you just have to head to your next flight without crossing any additional security checkpoint as if you just landed in SEA domestically.