I've travelled the other way (Vancouver to Orlando) a few times and it can be a challenge to find deals. One disadvantage for you is that May 20 is a holiday weekend in Canada (Victoria Day) and then the 29th is Memorial Day in the states so you hit them both! Is there any way to adjust your dates by a few days? Sometimes you can save enough by changing dates to cover the cost of a couple nights hotel and then you get a longer trip for the same price!
Overall though, I've had the best luck booking farther out rather than last minute. I've had good luck with Alaska Air and Westjet, although you have to look at the flight times carefully; they like to offer overnight flights that have you stranded in another airport for five or six hours in the middle of the night. ("Hey, that's a good price......no wait, it lands in San Jose at midnight then continues to Vancouver at five am!") Price alerts are great too, but I've had the best luck just saving my searches and checking whenever I happen to be on my computer. Familiarize yourself with the prices, know what you want and be prepared to book right away if you see what you want. Good luck!