Normally street meat and other mobile late night snacks are reserved for the carnivores but the Taco Truck on Alvarado outside of the Von's parking lot is the best one around. They serve up both veggie tacos and your traditional meat offerings (I hear the suadero is king). Every single one of my friends who has eaten at the 'Taco Zone' truck has loved it.
They normally arrive in the evenings and weekends and hang out till around 2am. The food is fast, cheap and made by a team of super friendly ladies.
side note: the salsas are fantastic