The Vegetarian Center on Mahidol road has recently changed it's format from a coupon-based food-center style system to a semi-buffet style, where you can pile dishes on to your plate yourself, but then go to pay the cashier for everything individually. So it's not a buffet in the 'all you can eat for a fixed price' sense. That said, prices are seriously cheap. Two vegetable or curry type dishes over brown rice go for 10 baht. And because you serve yourself, portions can be rather generous depending how hungry or modest you are. :-D
The Vegetarian Center has been in operation for a very long time, and is also firmly on the tourist/traveler map, it's in the Lonely Planet guide as well as some tourist maps of the city. I think the food is not quite as good as at some of the Chinese-Buddhist oriented vegetarian places like Mata Restaurant (http://chanchao.fotopages.com/?entry=67431(See link)). It's a bit hit & miss really. Also this place feels far more like an 'alternative' health-food kind of place. There's a small shop as well that sells things like buckwheat, lentils & lotus seeds and all the other things them macrobiotic yoghurt weaving health-nuts eat. 8-) I really love Thai 'Jay' vegetarian food but at times this place reminds me a bit of the vegetarian fare a rather radically 'green' auntie of mine used to make.. No arguments about how healthy it is though..
But I still want to mention this place, the thing that swung the balance back in favour were the very nice fried spring-rolls you see on the 4th photo here. They go for 6 baht (11 for baht for two), and you can take them home in a totally environmentally sound & sustainable 'hot dog tray' made out of banana leaves.. Just needs some ketchup and onions. :-D