For that many Avios, plus $35 for one of the more scenic train rides in North America, take Cathay Pacific's JFK-YVR flight in J or F. It arrives in the middle of the night, but if you've slept, you can drink coffee for a couple of hours, then make your way to the Vancouver train station and take the 6:40 AM train to Seattle, following along the coast while you have breakfast. Superb flight (best transcon in N. America, full stop) and a great train ride.

http://www.amtrakcascades.com/

Otherwise, JFK-SEA is a good FC service and it's quite a long flight westbound.