Finally, I went to a small shop which looked quite clean and there were lots of choices for food. I chose some eggs, some tiny shrimp paste, some stir fried vegetables, some stir fried spare ribs with sesame and some broth. I actually enjoyed my meal which cost only 20,000Vnd while speaking a few words with an old man from the countryside who drank a whole bottle of rice wine during his meal. He was hilarious with his strange accent and he invited me to drink wine with him, too. Oh my gosh, how can I keep up with him.
If you get lost in the Old Quarter at lunch time, go to 40 Cau Go street, cheap and good. Good luck!