After 1975 many Vietnamese settled in the Clarendon section of Arlington, Virginia, which became known as Little Saigon.
After a metro station opened there in the 1980s a lot of the Vietnamese businesses got priced out and moved a couple of miles up the road to the Eden Center, which is virtually 100% Vietnamese businesses, including some absolutely incredible restaurants.
Still some damned good Vietnamese restaurants in Clarendon but they tend to be more upscale. Eden Center is where you go if you want really good, but not at all expensive, Vietnamese food.