I think you may have a tough time changing in PDX at check-in as I don't believe AC is operating the station. If you can get away with no checked luggage it will leave you the most flexibility. I'd suggest checking when you get to YVR. If there is even a hint of delays explain how tight your connection is and ask to be rerouted.

There are clearly a lot of better routings, including Vancouver to London direct, Calgary or Toronto, but you'll need to convince them it is in their best interests to move you. If you end up in Montreal and you've missed your flight assuming flights are operating you should be able to get back to Toronto for the late flight to London, so all is not lost.

Good luck and happy travels. Remember to make the most of the various lounges on the way. As well as those in Canada, you'll really enjoy the London Lounge in London on your way home, it's one of my favorites.