I am not sure about Vancouver, but both Montreal and Toronto are very gay friendly, though from what I've noticed, gay-bashing seems to happen more frequently so in Toronto. Montreal is without a doubt one of the best tourist attractions Canada can offer, but with all the protests going on there, and also a whole lot of corruption in their construction industry, I am kind of skeptical to see it as one of the better cities to live in Canada.
Which leaves Vancouver and Toronto. As for gay life, from what I've experienced, Toronto definitely has a huge gay scene, but it's still not much to boast about. It's awfully cliquey, and more people are less willing to make friends and more willing to just fuck so it can be a tough place to make friends. If you are planning on doing a coop there though, I am sure you will make friends at work, and gay bars and clubs there are definitely worth visiting, as long as you have friends around to hang out with.
As for actually living in the city, if you can afford living in Vancouver, I would definitely recommend it. Great food, friendly, very liberal community, and also great climate! Vancouver has a pretty big gay scene too so you won't be missing out too much by picking Vancouver over Toronto. Being a Calgarian myself, I found people from Vancouver to be way more accepting and friendly than the locals in Toronto, who are pretty much like the Canadian version of New Yorkers.