I have walked past the jumbo bus, Taco "truck" at Othello and MLK, by the Othello lightrail station, countless times and finally stopped in for some tacos. There weren't any vegetarian tacos on the menu, but it was no problem to order them. After you order, you can actually board the bus and and sit inside and eat and watch Mexican TV, but I got my tacos to go.
I ordered three vegetarian tacos. They were somewhere around a buck fifty a piece, and came full of beans, rice, lettuce, tomatoes and sour cream. Both the red and the green salsas were delicious and spicy. I think the refried beans may have been cooked with lard.
The one thing that was obviously lacking was the little packet of pickled carrot/radish/jalapeno/fresh lime... on one hand, they are tiny insignificant garnishes, on the other, they are one of my favorite parts of eating at a taco truck.
These tacos totally hit the spot, and are hard to beat for the price. One complaint is the use of styrofoam in the packaging for the tacos to-go. I'd definitely be willing to pay a little more for a compostable option.