Thuy Cafe is a good place to get a quick banh mi, but the menu is pretty limited for vegans. As the description mentions, vegans will need to request no "shmear" on the banh mi, which contains butter. The rest is simply pickled daikon/carrot slaw, jalapeño, cilantro, and a mixture of stir fried tofu and bean curd skin. It's a bit dry, but it's tasty and fresh, and it's a deal at under $4. Several of the boba drinks are vegan (mostly the fruit ones), but you'll need to ask, as many contain dairy. The tofu com chay is vegan.
They're very friendly here, and the lounge area is a nice spot to enjoy a light lunch, so it's worth checking out despite the limitations.
Cons: Few vegan options, Often need to ask about ingredients