Finally got by this little stand in the Winter Street Arcade (where, incidentally, the original Falafel King is still operating, despite its fancy new storefront a few blocks away on Washington St). Seven banh mi options (nice), priced $3.99-$4.45, including what the owner told me is the most popular, crispy pork.
I don't think I've seen this particular protein before (it's basically a chicharron-like skin-fat-meat cut) in a banh mi, so tried that first. Very good: decent if not great roll, plenty of fresh cilantro, fresh sliced jalapenos (the other "spicy" option being sriracha), allegedly housemade mayo, daikon and carrot pickles, cucumber, sauteed scallions (didn't much notice this novel ingredient in there), and Maggi. Nice crispness to the pork rind.
They also do fresh rolls, seaweed salad, miso soup, bubble tea, and Vietnamese iced coffee, though the latter was gone by the time I visited late in the lunch cycle.
Chinatown is closer to work for me (based on a very recent visit, I can report that Mei Sum's pork charcuterie banh mi is as terrific as ever, the Globe's off-kilter roundup notwithstanding), so I can't imagine getting back here a lot, but it's a worthy addition to the growing banh mi selection around town. I think I'll try their meatball next time, which I assume will be in the "juicy" style (crushed into a Sloppy-Joe-like spread).