Well, I try to avoid fast food, but in a pinch -- Chipotle can make up a lovely vegan burrito. It isn't cheap, but it's wonderful. Get the black beans and avoid the pinto beans, though, the pintos are made with bacon.
Qdobo also has vegetarian options, but I don't trust them as much coz the chain is owned by own of the big food conglomerates (Pepsi, I think).
Smaller places, like a local Indian restaurant with a lunch buffet are often better . . . but, you have to ask questions, especially if you're vegan. And that can make it less than fast!