you can often set up an alert on many sites so when the price hits a fee your willing to pay it will alert you.
we have friends who come every year and christmas day is usually the cheapest, they always kind of get a kick out of walking on the beach on christmas day.
if you an leave earlier like dec 6-10 you will pay around 320 one way.
i do this flight around 7 times a year and united or AA s the best either thru hoston or dallas, if for any reason your persona no gratis in the USA well than your gonna pay big time as direct flights are way more expensive.
if you take the 630 from toronto to houston and then connect at 10 am ish you can be in vallarta around 130.