How about Harry's Tap Room? They have locations in Clarendon and Pentagon City, which are both metro accessible. It looks like there are some vegan options on the menu, too.

http://www.harrystaproom.com

Some other thoughts, all in Arlington near metro, but not sure about vegan choices:

Carribbean Breeze in Ballston
http://www.caribbeanbreezeva.com

Liberty Tavern in Clarendon
http://www.thelibertytavern.com

Jaleo in Crystal City
(Serves tapas. 3 stars from Tom Sietsema, celebrity chef with great reputation)
http://www.jaleo.com