When you talk about vegetarian food in Thailand, it could be interpreted in more than one way. Thai vegetarian food would imply that you want, Thai food dishes that are made in a vegetarian style. On the other hand the term vegetarian food in Thailand, could cover all types of vegetarian food even if they are not related to Thai food. For example, an Italian vegetarian Pizza is not a Thai food dish but, you could eat it in an Italian restaurant in Thailand. The fact that most Thais are non-vegetarians means that vegetarian food in Thailand is not very common.
If you plan to visit Thailand and require vegetarian food, make sure that you make plans for this. For example, if you are going to stay in a fairly large hotel in the main cities like Bangkok, Chiangmai or Phuket - explaining what type of vegetarian food you need would not be a problem. The big hotels also do not have limitations related to language, so they can comprehend your request well. Things would be a bit different if you were travelling upcountry as both language and the lack of vegetarian food can pose a challenge.
Some restaurants would agree to customize their food dishes to make them vegetarian. It is not uncommon for larger restaurants to have a small section in the menu dedicated to vegetarian food. At the time of ordering your vegetarian food in Thailand make sure, that the staff understands what you need. It is not uncommon for restaurant staff to ask half a dozen questions related to vegetarian food! The common questions could include, whether you can eat fish sauce, whether eggs or fish are okay etc. The stricter you are regarding your vegetarian diet, the more careful you have to be when explaining your requirements.
We have published two resources to help you with vegetarian food requirements in Thailand. The first report http://www.leisure.food-recipe-cooking.com/vegetarian-food-thailand-frc-vg-290412.htm presumes that you are a visitor, it goes on to explain how to request for vegetarian food according to your preference. The other report http://www.thaifood.food-recipe-cooking.com/thai-vegetarian-food-dishes-frc-vg-01.htmhttp://www.thaifood.food-recipe-cooking.com/thai-vegetarian-food-dishes-frc-vg-01.htm relates to Thai vegetarian food and not just any type of food that is vegetarian. Between these two reports, you would have developed many important skills to get the right vegetarian food on your trip to Thailand.