If you eat a strict diet of vegetarian of vegan food, traveling in Thailand or living in Bangkok, and eating local food can sometimes be a challenge. Many normal Thai dishes are filled with small amounts of meat, and even if meat isn't actually in the dish, there's often fish sauce or meat broth inside of some dishes - so that just goes to say that it can be a challenge to find good and authentic Thai vegetarian food (more tips in my vegetarian Thai eating guide: http://www.eatingthaifood.com/vegetar...)
In order to be certain you're getting purely vegan or vegetarian food when you're in Bangkok, you really need to look to some of the exclusive vegetarian restaurants. There are quite a few scattered around the city, but one of my favorites is a restaurant known as Chamlong Sandi Asoke (จำลอง สันติอโศก). It's really more of a food court, where there are a number of stalls where you can choose what to eat and sit down and eat them all. As soon as you arrive at this vegetarian grazing zone, you'll need to exchange your money for some coupons. I usually exchange about 100 baht per person, just to make sure I have enough money to get a few different dishes - believe me you'll want to try a few different things when you eat there. The great news for you, that is if you eat vegetarian, is that everything in the food court is exclusively vegetarian (and vegan), so you don't need to worry or even ask what they put in their cooking.
There are many different vegetarian Thai dishes to choose from when you're at Chamlong Sandi Asoke (จำลอง สันติอโศก). There are a couple of places that seem to specialize in fake looking meat, that's really made from protein flakes or things like that, and then there are plenty of other vegetarian dishes. I normally like to start my vegetarian feast with spring rolls, and you can either choose to get the deep fried version or the fresh non-fried version - both are good, depending on how you're feeling for the day. You can then branch out and try some Thai vegetarian curries, or other dishes like grilled mushrooms, vegetarian noodles, or even vegetarian pork skewers (don't worry, even though it may look like meat, it's not real meat).
One of my personal favorite vegetarian Thai dishes is tam ponlamai (ส้มตำผลไม้), or fruit salad, and I think they make a really tasty version at Chamlong Sandi Asoke (จำลอง สันติอโศก). The fruit salad includes a mixture of guava, rose apple, dragon fruit, watermelon, and a few other fruits, mixed with soy sauce, lime juice, lots of chilies, and some crushed peanuts. The result is an incredibly refreshing fruit salad that has a beautiful contrast of flavors, including spicy, sweet, and sour - I really enjoy it. If you go to Chamlong Sandi Asoke (จำลอง สันติอโศก), don't forget to try their version of Thai fruit salad. Another interesting vegetarian Thai dish to try is kanom jeen nam prik (ขนมจีนน้ำพริก), or white rice noodles served with a peanut based curry on top. It's rich and creamy, and if you love noodles, you'll probably love this dish.
It can be a challenge to find good and authentic Thai vegetarian food, unless you go to the right places. Chamlong Sandi Asoke (จำลอง สันติอโศก) is one of the best restaurants for vegetarian food in Bangkok, and you'll find so many delicious things to try.
How to get there: The restaurant is located very near to Chatuchak Weekend Market, and directly behind the IN Square shopping mall. There's a small road to the right hand side of IN Square mall. Walk through that street and at the end hang left and you'll see a small pedestrian gate, enter through the gate and you'll be at the restaurant.