I went up to Montreal towards the end of May a couple of years back & had a great time. Other than the fact that it's the second largest predominately french speaking city in the world, after Paris & quite multicultural generally - no one single thing really jumped out at me but I simply found the city quite enjoyable overall. You will likely enjoy the city more if you speak Francais - or at least make the effort to learn a few basic words & phrases. I definitely suggest visiting if you can!
There are a good number of vegan & vegan-friendly places around. Check out: www.happycow.net/north_america/canada/quebec/montreal/
Aux Vivres seems to be the mot popular vegan place. I mostly ate with the folks I was staying with & cheap falafels!
My visit was centered around trying to be a temporary local & I generally shunned "attractions" in favor of wandering about the city trying to get a sense of what it would be like to live there. I particularly enjoyed included hiking up to the top of Parc du Mont-Royal, walking around Boulevard Saint-Laurent + the area between there & Rue Saint Denis. Vieux Montreal, the oldest part of the city is down by the river & also neat, if you do your best to ignore the touristic focus. Marche Jean-Talon is a neat open air produce market surrounded by small shop, worth a visit in itself plus you can then wander around the surrounding neighborhood of Petit Italie.
My personal favorite place to people watch was the extremely multi-cultural & working class neighborhood of Parc-Extension. Also, while there are tons of nice little markets around Montreal but I found the best prices & selection of many vegan products at the Loblaws chain supermarket just next to the Station Parc metro stop in Parc-Extension. I think this is also the first stop for the longer distance commuter trains heading out of the city towards the Laurentian Mountains & the Parc du Mont-Treblant provincial park.
Personally, I suggest sticking with public transportation & walking for the city itself. If you go further afield than a car would likely be handy though you can take Amtrak or Greyhound to Montreal from the states & once up there you can take Via Rail, do ride shares &/or use the regional trains & buses to get around too.
I hope this helps! Please share pictures & stories if you do go!