In Hanoi, people are more used to eating at home. After work, people hurriedly go to the market to buy some food and go home, cook and eat with their family. This is a good thing to do, but it's time consuming. However, a popular dish to have out of the home in Hanoi is Pho Ga (Noodles with chicken), the special point of which is that people can eat it as a snack or as the main course, and the location is not a problem at all.
I think people have started eating out for lunch more now in big cities, or they're prepared food for the whole day in the morning, so that they can save their time and they still can enjoy their meals at home. Vietnamese people usually take a short nap after lunch and that's why there is little action on the street at lunch time.