Wishing you the best of luck, George and will watch your progress with interest. Cold isn't the only limiting factor, though. If you can keep the trees alive and growing, it's very doubtful that you get enough warm, sunny weather in Vancouver for it to flower. Then there is pollination, which is a complex process for avocado even in the best of conditions.
As to the little plastic pop-up greenhouses, they really don't last very long and aren't tall enough for avocado which tend to be tall and narrow. The seams tear, the zippers fail and the clear plastic lasts about 2 years before it gets stiff and cracks and tears. I stored them carefully indoors, and only used them in the winter months, too. The steel framework is also very flimsy. I've had 3 of the smaller shelf unit ones, used to keep my orchids in them in winter. Now, I have a hoop house structure that I cover with frost cloth attached to a south-facing wall of our big garage. This is a better, and more durable option, imo. A south-facing exposure is certainly needed for an avocado growing there, anyway to get the maximal sun exposure.
A big, fancy greenhouse with glass, ventilation fans and supplemental lighting might do the trick long term, but you still have the problem of pollination. Most avo's must be pollinated by a different variety but they must bloom at the same time for it to happen. Wind plays the role to disperse the pollen. My tree was pollinated by a neighbor's a half a block away, but upwind. I got good fruit, but his tree hardly bore at all because there was no other tree upwind of his. As I said, it's complicated. I only got fruit every second year, too. Many avocado varieties have an "on" and an "off" year.
