I concur with Tango - Whenever I have transferred US-International in YVR I have always had to clear customs but it may work for you.

However I am intrigued as to why you would go via YVR rather than direct on NW out of SEA. Normally its better to fly YVR to SEA to take advantage of the better sale fares out of SEA. However YVR would be cheaper if you are flying regular Biz (C/D but not Z which is cheaper out of SEA)

btw I really reccomend the Van Gogh exhibition while in AMS if its still on.