Here is a Chiangmai Blogger's travel menu: http://www.nachang.com/travelmenu/thai_travel_menu.pdf Although it is for general dining, Page #2 gives some good info about vegetarian, including the Thai script for "jay".
Here's another resource: http://www.happycow.net/asia/thailand/ Between Bangkok and Chiangmai, there are over 150 listings.
Surprisingly, Bangkok has quite a few "Thai" restaurants. ;-) Although we are not vegetarian, we found a great vegetarian stall at United Center Silom's 3rd floor food court. If around Silom, during weekday lunch, it might be worth a stop.
re: Curt the Soi Hound
I don't think the HappyCow site is up to date, but I do agree with the recs in Chiang Mai for PunPun (at Wat Suan Dok), Khun Churn (100 B for the lunch buffet for about a year now), and Aum on Moon Maung Road.
If fish sauce is not a problem, almost any restaurant will have a variety of vegetarian dishes in Thailand.
Full disclosure: I'm not a vegetarian
I'm heading for Hanoi in 1.5 weeks, saw this Yelp-esque site for expats that included a few vegetarian restaurants: http://newhanoian.xemzi.com/venue/cat...
Went to Chiang Mai and Bangkok last year, I don't think it'll be much of a stretch to find vegetarian dishes. In Bangkok, we mostly ate off the street though, I didn't really see many Thai restaurants.