Whats the cheapest month to fly to Vancouver, Canada?

Probably late October to early December, and again from early January to late April. The very months when no one wants to visit because the weather's lousy everywhere in the northern hemisphere and it isn't Christmas when everyone wants to visit relatives. But really, the best thing to do is keep checking airline or travel websites until you get a pretty good idea what the prices usually are and you know a good deal when you find one. Sometimes you can get an extra good price if you book and pay well ahead of time, sometimes you get a good deal by booking at the last minute.

It varies widely, according to tourist demand, etc.etc.etc. There's no way of predicting when there will be a seat sale - your best bet is to contact one of the international carriers - Air Canada or one of the British airways offices - and find out if there are any seat sales coming up