you can take your A-C vouchers to any travel agency in Alberta and they will process them, you do not have to deal directly with AirCanada.
the downside to your situation: AirCanada flights from Calagary are limited so your choice of resorts has been reduced somewhat. as for booking a hotel you have 2 options. you can choose one that
AirCanada Vacations offers or you can book through
Red Seal Vacations. the second choice prices' are land only(no airfare) and by the day, so that if you are adventuresome you could get to see more of the island.
and what john says is true, service and reliability are not significantly different from one charter company to another. the only measurable difference is in the airline they use/own to service their destinations.
mob