This was a convenient place to get lunch or dinner back when I was working or taking classes in the area. Unfortunately as a vegan I was usually limited to a boring bean burrito ... though I also branched out into a bean tostada and a potato taco (though I did have suspicions that it had butter in it).
I loved the beans and tortillas, though, and as boring as it sounds I was always happy with a side of corn tortillas (specify no butter) and a side of beans (specify no cheese). For dessert I'd have an apple burrito -- very tasty, but I admit I never had the guts to ask if it was fried in the same oil as the meat dishes.
You can get better vegan or vegetarian Mexican food elsewhere, but if this is a convenient location for you, you will be able to find substantial meals.