The best time to purchase flights to Vancouver, from across Canada, is about ONE MONTH prior to your planned travel dates. It may be safe to purchase your ticket early, but there are always seat sales due to the high frequency of flights. The cost will increase one to two weeks prior to your planned travel dates. The range for a return ticket from Toronto to Vancouver (if purchased on sale, at the right time) is approximately $400-$600. The high season is during the summer months and around Christmas, including other statuatory holidays.
TIP: Use an online flight tracking tool. Plan ahead, research, and track early. These can be used for travelling everywhere, not just across Canada. Yapta.com is great because it is affiliated with several airlines and it emails you if your flights drops in price.
Vancouver Savvy 
What a great resource!
Terrific work! This is the type of information that should be shared around the web. Shame on the search engines for not positioning this post higher!