The reason this post is not called "Vegetarian street food in Hanoi", is because such a thing pretty much doesn't exist. And I know it's not very "friendly" to eat the fishies fermenting in your fish sauce, but you're hard pressed to get any fish sauce-free Vietnamese food, and it's damn near impossible if you want to try street food in particular.
So these suggestions come from my "best efforts" approach to being vegetarian in Hanoi, and will hopefully prove useful to any fellow vegetarians who don't want to be permanently relegated to the city's (still excellent) vegetarian Buddhist restaurants.
Handy hint: Much more successful than trying to say you’re “ăn chay” (vegetarian) is the phrase “không thịt” which means “no meat”. If you can’t pronounce it, then write it on a piece of paper and flash it around with wild abandon.
Another important piece of veggie vocab is “x́ dầu” which is “soy sauce”, which you can ask for as an alternative to the ubiquitous fish dipping sauce. Many places don’t have it though, so if you’re super dedicated you could always carry in your bag a few of those little plastic soy sauce fish you get with sushi. You can try asking for no fish sauce (“không nước mắm”) in the cooking, but good luck with the results!
Phở không thịt: Pho noodle soup with no meat