Okay, I've now dined at Juicy 4 U (best Khao Soi yet - make sure to sub rice noodles for the egg) Gap's House (highly recommend the experience of their nightly 6-dish vegetarian buffet and getting there at the start, 7pm) Blue Diamond (vegan pancakes! I had an apple cinnamon one yesterday, and a soy latte today), Aum, and Bodhi Zen, as well as attending a cooking class (and had no issues being vegan once passing on fish sauce and chicken when signing up and as the class vegan) at Asia Scenic Cooking School. I'll probably become a broken record on this, but it was a pleasant surprise that all four varieties of curry pastes the class made were naturally vegan.
Here's a great blog post on vegetarian options (largely vegan): http://www.neverendingvoyage.com/chiang ... staurants/
Some are a bit dated, as restaurants change and close.
For example, May Kaidee's in the Old City is now Morning Glory. It also offers cooking class, but you need to give them advance notice. Taste of Vegetarian, which also ran a cooking school, has now relocated to the other side of the moat as 'Vegetarian Hut', but I haven't been able to find it yet, since no actual street address is given.
I also combed the Chiang Mai Gate Market for the vegetarian stand (as well as the Sunday Walking Street food vendor coves), but no luck yet.
Next on my list: Khun Churn, Pun Pun, Freebird Cafe and a bowl of Khao Soi at Aum (enjoyed that on my last trip).
There is 'vegetarian' signage EVERYWHERE. It often does not mean 'no fish sauce', so make sure you ask about it.
And I'm randomly staying at a juice bar/guesthouse with a vegan burger. I wish the toilet flushed vs. using a bucket, but the fact that 'vegan' is on the menu helps make up for that.