This place is located in the Hoa Binh Shopping mall, somewhere between the McGowen and HCC/Ensemble Metro stations along Milam/Travis/Smith, I cannot remember which street exactly (apologies).
You have to enter the shopping complex on the south side: don't be put off by the disgusting smell that hits your nostrils at first, as that is due to the restrooms which are right by that door. They're pretty badly maintained...
The Original Givral's Sandwich Shop is straight down, and it's spartan to the point where they do not even have a proper shop sign: they simply have a banner above the door that says "The Original Givral's Sanwich Shop"! Almost everyone there is Vietnamese, bar the occasional police officer who walks in (why do Vietnamese sandwich places always have HPD officers around?)
I had the Barbequed pork Banh Mi, (two of them, actually). The meat is grilled and marinated to perfection, in large part due to the selection of the meat: I peeked into the sandwich, and suffice to say that if you're on a low-fat diet, don't look.
The sandwich is filled with slivered carrots, sliced cucumbers, cilantro and jalapenos. It's everything that you could ever want in a sandwich: delicious meat, crisp and crunchy fillings, and the bread is freshly baked, and has a much better texture than Thiem Hung No.3. It's a lot denser as well, weighing like a brick.
I also had the fried plantain (actually fried banana), which is battered and deep fried, together with a sesame-ball with mung-bean filling (this was good, but very filling: the mung bean paste was not as sweet as some others I've had), together with their Vietnamese coffee, which was wonderful for two reasons: they use decent coffee grounds, and they serve it to you with the water separate from the percolator, so you can add as much water as you want to add.
All in all, this is probably the quintessential chowhound hole-in-the-wall experience. I think their sandwiches are the best in Houston, without a doubt.