It's not easy to be true to a vegan diet in Thailand. But I found a number of pleasures. Breakfast bought on the street is not bad, with a lot of fresh fruit, sticky rice, coconut, soy milk, and Chinese 油條 offered. I particularly like the sticky rice with coconut milk roasted in a banana leaf wrapper over coals.
For dinner in Bangkok, I strongly recommend the expensive restaurant Bo.lan. The food is excellent, and the Australian chef is friendly and obliging. There's a vegetarian set-price menu that's very fine. I added a glass of fancy rice wine that was okay. Desserts went on and on.
In Chiang Mai, head to Reform Kafe. It's largely patronized by foreign tourists (Thais are not vegetarians), but the food is honest and delicious. The servers speak English well, and there is a long menu. The restaurant's located in Green Tiger House.