Jack's - 6th st - just south of H - right before MCI arena.
Place looks horrible, isn't comfortable to dine in, but has fantastic Vietnamese food. And it's cheap! Lunch is $5.
I would have never gone there except some Vietnamese people I work with said it was pretty good.
I get honey glazed bbq pork or sesame bbq beef - both always good. Just get it to go if you can...