In Union Station (right by the Capitol) there are 30 some-odd "fast
food" places, including a vegetarian Indian place. It is not ff, but
the food is great and cheap.

In the Adams Morgan neighborhood near Columbia Rd., there are about 3
or 4 Ethiopian restaurants with veggie entries on the menu. This is
some of the best food on the planet!! I especially recommend
Mescarem, 2434 18th St., NW. The Red Sea is right across the street
and is also excellent.

In the DC area there are many Indian restaruants, specifically
vegetarian Indian Rest. In the Old Post Office Building on
Pennsylvania Ave, there is a very neat, small veg. Indian fast food
place with wonderful menu.

Understand, none of thes are FF places, but, it's worth it.

Good luck.