When you mention Tijuana Flats, I assume you are in or near FL. Charlotte, NC had a Tijuana Flats for about a year and I loved that place. I really miss it. Definitely the best high end fast food place with veg options that I have seen. By high end fast food, I mean you actually get a platter of food, not paper wrapped individual menu items.
I had to look up the Taco Bus and I was impressed with their menu's vegan options. I don't know when I might get back down to FL, but next time I go, I'll make sure I find them. FL really impressed me with their vegan options when I visited about 2 years ago. My fav down there was Hakin's Vegetarian Restaurant.