I am vegan for 15 years and know Thailand quite a bit: I state that May Kaidee's on Thanon Samsen is the best affordable vegan restaurant I've ever been.
Their vegan pad thai, for instance, is truly a marvel; it was better than on their Tanao location.
The service was great too (Feb-March 2013): the waiter, a young man, was always very friendly and spoke english well.
I can't support the comments saying the portions are small, the food mundane or the service perfectible: we went to this restaurant a few times during our trip and there we've got the largest portions of all our trip, the best service (besides the "31" in Krabi), and by far the tastiest food.
All in all the best value for money, hands down.
Cons: Not spicy (or authentic) enough