Tan Tan is my favorite vietnamese place in houston . it is right in the middle of the vietnamese area in houston near the beltway and bellaire blvd. i heard they opened another one on westheimer closer in but i have not been there.
I'm also a fan of Van Loc. It is also really good. It is near mid town.
Pho Saigon is also supposed to be good. locations around houston. i havent been there but i hear its good.
Houston has a large vietnamese community and some really good places everywhere.