I lived in Chiang Mai for 6 months back in 2017 from May till October. Chiang Mai is a great place for vegans as there are many restaurants and buffets available that are really cheap! Theres a jay symbol to look out for in Thailand which pretty much represents vegan food (without onion & garlic). Ive eaten out at most places and have concluded a list of my favourites. If you're ever in town then you should check them out!
Imm Aim Vegetarian Cafe do a pretty decent pad thai for 65 baht (order extra serve for 15baht- I usually get 2 extra) as well as having a good range of salads and Thai massaman curry. I always end up getting the pad thai, but I think they recently started selling vegan baked goods and sweets as well which is pretty cool. The pricing is cheap for western standards and the owner "O" is also vegan and a cool guy to talk to as they speak english pretty well.
The Jay buffet on the bottom floor of Maya mall is an easy option thats very cheap- a decent plate of food costs about 60baht, which you can get two of if your carbin up. I ended up bringing my own container of rice and filled it with toppings that cost 10baht each (and theres several oil free options too!). Also, the spring rolls here are the best and only cost 10baht each! I went to this place often as a lot of vegan crew ate here most nights since we all lived close by and theres a burrito place opposite the buffet that do tasty vegan burritos for 80baht which is neat.
Dee-J Restaurant is one I recently discovered before I left Chiang Mai and I really loved this place as its cheap as and the papaya salad is freakin amazinnggg. I always ended up ordering the papaya salad for 50 baht plus had around 6-8 servings of rice that were 10 baht each. The curry there is also really good and the dishes are all vegan except for the yellow egg noodles. (pic is pretty low quality as I took a screenshot from a video).
Mai Kaidee's is also a really good place. The food has a lot of flavour and is proper thai food instead as the chef is from down south. The massaman curry there was my favourite and I ordered rice as well. Every time I ate out here I enjoyed this place, as well as my mates whom I ate with. The dishes average at 60 baht which is pretty decent.
Happy Green is an all you can eat vegan buffet, located about 7 km from Maya mall- which is quite far away from the square. It is decent price at only 140baht and theres usually plenty of food available thats all vegan and pretty clean. They grow their own veggies in the back garden which is cool and the dishes are generally pretty tasty. Its a nice place, quiet setting with a temple in the background and free wifi to chill as you can eat there for however long you like.
Lastly is Aum. I have to be honest this place isn't one of my favourites but a few people I know liked the food there and its good to try it out if you're visiting. The prices here aren't exactly cheap, but I rate the dumplings and burger. I wasn't too keen on the amount of tofu they put in the food but its algood. The setting is really nice as you sit around a big table and can lie down after having a massive feed.
Theres other places too such as Taste Of Heaven, Blue Diamond and Amrita Garden which I heard are really tasty, especially Blue Diamond which is a vegan bakery with many tasty goods.
So yeah, thats all from the top of my head, if you rate any other places let me know! :)