Hi all you Vancouverites! I'm a born and raised in W. Van person who now lives in Florida. Gotta tell you, my mother used to sprout avocado seeds all the time and grow them in the sunny corner window of our kitchen.
Don't want to discourage you, as I do understand how much fun it is to grow these. But even if you can keep them alive through the occasional freezes you get there, you very likely won't ever get fruit from any of them. From seed it takes often 10 years even here in Florida for an avocado to bear anything. If you want to grow your own for fruit, you buy a grafted plant of a good variety, then it only takes maybe 3 years for fruit. Plus, we have at least 3 times the length of growing season here plus very little if any cold weather, barring a few nights that go down into the 40's. Pollination is another problem as it's a sort of complex process and some types don't self-pollinate, they need another tree close by for pollen.
My advice would be to grow them as nice foliage plants, move them outdoors in the summer as long as the nights are warm, and the rest of the year keep them indoors. They grow into tall, narrow trees, so the shape is actually fairly nice for a large house plant if you have room for it, and VERY good light - a south-facing window is necessary for your short, gloomy winter days there. I'd be inclined to use a grow light on it, for a few hours each day through the winter months, too.
As George wrote in the original post, they come back from the roots even if they're killed back at the tops by cold. So if you want to keep it small enough to bring indoors, just cut it back regularly.
