There is a hidden gem which not a lot of people know about, and not a lot of people go out of their way to find it. On Suthep Road there is a vegan place called Jay Mai Jam Jay, all the food is delicious and they have the hugest menu, and their vegan wonton soup is to die for. It's a local place, so the prices are cheap, but they have an English menu.
If you walk south along Nimmanhaemin Road (away from Maya), when you get to Suthep Road cross the road and turn left, keep going for a bit until you're just about opposite the CMU School of Pharmacy. There you will find three jay places side by side - Jay Mai Jam Jay keeps longer hours than the other two so usually it'll be the only one open. It is a long narrow restaurant with pictures of the food and numbers all over the walls. :) Enjoy!