Passengers from JFK are let off plane in YVR for the hour+ layover. From the gate, you can go upstairs to the CX lounge. At that late hour, it is unlikely the Cdn Empress Club will be open, since CX is the only flight heading out from YVR at that time of night. Great food, as mentioned, but at that hour and after a meal enroute from JFK and another once you leave YVR, even dim sum may be too much to face. (They don't have any computers, you'd have to set up your laptop in one of the phone carousels.)
At JFK, the Terminal is the BA/UA Int'l one and the lounge used is the BA Executive Club on the second floor. This would be the only one open to J and F passengers, and oneworld elites travelling in Y class. There is a back door down to the security check and gate. The Admiral's Club is in AA's terminal next door, and not the one you'd use since CX does not operate from that building.