Sunwing is the only (relatively) cheap option YVR-YYZ, but I would investigate their reliability before booking with them, even if you
could book that is.
Jazz doesn't fly YVR-YYZ, only mainline AC.
Travel in Canada is no longer cheap. The Westjet and AC fare wars are gone, sometimes they have sales which usually last for just 1-2 days and each matches the other anyway. The train ain't cheap unless you want to sit up for 5 days
(2 of which are mind-numbingly boring).
I would not recommend the SEA-BUF option either, by the time you add in getting to/from either it won't save much if anything. Plus you have to deal with the US 'authorities' (ICE and TSA), not conducive to a good holiday experience.
Check out airfarewatchdog.com (I don't think that's a commercial no-no) and sign up for notifications ex-YVR. They seem to pick up the transitory deals quickly enough.