Everyone who has lived in Amherst knows of Bueno Y Sano. They are a classic budding business-- great food so great, they expanded to Northampton and Springfield in the past 5 (or so) years. Vegan options: bean burrito, salvadorian (bean and veg) burrito, chick pea burrito, seitan burrito (oh, there are salads but I've never had 'em!). They will put yogurt or sour cream or something in all burritos by default, but they are the most customizable place I've ever been to... whatever you want, you can get. I always say "no dairy, add guac" so that the yogurt and the cheese gets left out. Guac is sooo good.
Cons: can get busy