Unless you plan two separate awards, you can't book flights between two Canadian cities via an intermediate US city. You would require one award between YYZ-JFK-YYZ and a second on CX between JFK-YVR-JFK. It is unlikely you'll find premium seats between JFK-YVR-JFK at this time of year in advance but F and some J often open up within a week of flight time. Not sure Y+ seats are even available as awards...nor is this class of service available on CX flights with F cabins on this route.