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Vancouver is the home territory of Telus, so they're probably your best bet. All three of our major carriers (Rogers, Telus and Bell) support GSM and carry the iPhone 4, so you should be fine.
I have heard Rogers and Telus are good but I have never been to CA and I am unfamiliar with their plans. With all wireless coverage it all depends on the area. I would recommend talking with someone when you get there and finding out what is good in the area you are going to be visiting.
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