Well this is a huge authority on this:
http://www.vegdc.com/restaurants.php
I love a lot of places in DC - are you looking solely for fully veg places? Not knowing what kind of foods you like or which you have been to, I will suggest:
-Sticky Fingers, a vegan bakery that also serves some savory stuff. Located in Columbia Heights. I highly recommend the Club sandwich (vegan), the vegan chicken nuggets with barbecue sauce and all of the cupcakes/baked goods
-Everlasting Life, located near Howard University. Waited way too long to go here! The most amazing soul food, all vegan. Love the kale salads, tofu skin salad, macaroni salad, lasagne, philly steak sandwhich, chicken sandwhich, fresh squeezed juices etc
-Vegetable Garden or Yaun Fu in Rockville. VG is closer to Bethesda on the Pike - Yaun Fu is closer to Rockville town square. Generally we prefer VG but both are good. VG has amazing wonton soup and a huge vegan menu of every veg take on a chinese dish you can imagine.
-Woodlands in Langley Park. A hike but omg so worth it. They have a veg buffet on the weekends for brunch for $12. Most amazing array of authentic and veg indian cuisine
-Java Green downtown is a must-visit at least a few times. Lunch kind of place with lots of veg sandwhiches and asian dishes
Further out are:
-Great Sage, an amazing place that is a hike up north. Amazing place though.
-Sunflower in Vienna is amazing veg chinese food.
I don't know if you are also interested in veg-friendly suggestions so I will only offer one unless you ask for more
-Founding Farmers has a special vegan menu that you have to request. Delicious!