Banh mi is one of the better known Vietnamese dishes in the US (aside from pho). In Hanoi, Banh Mi 25, a small street-side stand, is known to have the best in the city.
Each banh mi is toasted to perfection and perfectly delicious. Depending on how hungry you are, you can either eat one as a snack or make it a meal. My favorite is the BBQ pork with pate for VND20,000. There are vegetarian (not vegan, though) banh mi options as well.
Grab a seat at one of the small tables the owners put out and enjoy your sandwich with a small glass of complimentary tea and watch the motorbike traffic go by.