Eating vegan or mostly vegan in D.C. is pretty easy. Everywhere we went restaurants offered plenty of vegetarian options and or devoted pages on the menu. Servers were pretty cool when I would ask "is there anyway I could get that veggie dish without cheese?"
Of course lots of veggie dishes are made with butter and or some sort of light cream. When this was the case, I tried to customize what I could without dissecting the entire dish and making the server want to slap me in the face. ;)
My favorite place in D.C. was hands down Amsterdam Falafelshop. Holy gomukhasana, FREAKN AMAZING! Can't wait to go back. I'd honestly eat there for every meal and never get sick of it.