For you, because you're travelling from Vancouver (which is where I am too) to Mexico, you're best bet is to get a travel agent to help you out. You're more than welcome to ask me any questions if you're not currently working with someone.
What you can do is have the full all inclusive package (flights, transfers and all inclusive accommodation) quoted through a Canadian tour operator, and then you can have land only pricing quoted as well which your U.S guests can use.
Most of my Brides have their guests contact me, and I take care of all the bookings for them. For May 2011 you would need to have your guests pay a deposit of anything from $150-$200/person to hold their spot and then final payment will be due around 45 days prior to the departure date. That is unless you get an early booking bonus built into the pricing, which may ask everyone to pay earlier if they want the deal. There's also going to be other promotions that the tour operator will offer you (upgrades on flights, free rooms, resort credits,extra baggage, etc). This is something that a travel agent can negoiate for you, so you're getting the best deal, best flights and best promotions possible