This is a delightful little Jay buffet restaurant close to the centre of Sukothai. I found it during the evening and they seemed semi-open with a few dishes available, but it is clearly mostly a breakfast and lunch place.
One staff member (there were only two) spoke some English, and was keen to talk to me.
Being a Jay place, it should all be vegan, and this seemed to hold true. Some dishes were hot as to be expected, and some weren't. A filling meal did not cost much.
I will upload a map showing where it is which was provided by Garden House, a good guest house I stayed at, with a simple but comfortable (fan) room for 150 Baht, a short and pleasant walk from "Jay Jay".
This may be your only veg meal between Bangkok and Chiang Mai.