I have a new favorite in the Seattle taco truck department: Flair Tacos in Fremont. This is vegetarian's taco truck dream with two awesome, intentional, distinct types of vegetarian (appear to be vegan) tacos: a grilled veggie version, and a tofu adobado version. Both of them are delicious.
Two grilled veggie, two tofu adobado.
In addition, Flair makes tasty salsa and adds plenty of radishes and limes to your order. It is also open late, something that I have benefitted from after a celebratory night in Fremont, but the tacos are equally delicious in the sober light of day.
It's not hard to find a decent vegetarian taco around town, but the Flair Taco truck has really taken the prize for creating an intentional, healthy option for vegetarians that looks and tastes every bit as satisfying as the meaty counterparts. As a general indication of taco size, if I'm super hungry, I can probably eat around 5 tacos. A typical light meal might be 3-4 tacos. One taco makes a perfect little snack.
Prices are typical taco truck cheap.