In Hanoi, you need to be on your guard. There are several types of annoyances, from overeager postcard sellers, to insane motorists, and finally, young teenage boys with nasty ideas in mind. We ran into several of them in a single night.
Post-card sellers and other vendors operate on the streets, especially in the Old Quarter and around Hoan Kiem Lake. They are very eager to sell you maps of the city and postcards of Vietnam. These postcards are not of the best quality, but since they are very cheap, I recommend you buy something. You are encouraging entrepreneurship and you might even have a fun conversation with the young boys and girls that do this. Unfortunately, if you've already bought your quota of postcards and are merely trying to get on your way, some postcard sellers do not take it so well. For us, the worst thing that happened is that the boy shouted very loudly in our eyes that we were fat. I thought of several clever comebacks much later.
Several people have written forums about crossing the road in Hanoi. It is indeed harrowing. You really must walk across slowly and hope you do not get hit. At the same time, the sidewalks are covered with parked scooters and street vendors. You will have to walk in the street at most times in the Old Quarter. People will even back their motor scooters into you, and my response to this was putting a hand on their back to let them know I was there. It's amazing to watch three or four people, including babes in arms, riding on a little scooter.
The only real danger we encountered was when we were taking an evening stroll around Hoan Kiem Lake. It is not very well lit, but there are many other people young and old strolling around the lake. After passing the bridge to the pagoda, a young man came up and asked our names. He told us he wanted to practice English. All seemed normal and then I glanced behind us. 5-6 young men were following us, and getting closer. I put my hand on my partner's backpack and gave them a look. Very shortly thereafter, the young man who wanted to practice English with us said very quickly that he had to go and all the boys ran off. I'm not sure what their plans were, whether to mug us, pickpocket us while we weren't looking, or maybe take advantage of us in an even worse way. I am just glad I was alert and aware.
That said, the next day, we met two lovely young college girls and had a great conversation with them about Vietnam and America. I compared Boston to Hanoi, saying it was where our revolutionary war started, when we threw off the English and declared ourselves independent. While we talked to them, some postcard sellers approached us, but other than that, I couldn't detect any scam. So don't be suspect of anyone who wants to practice English, but be aware of what is going on, particularly if it is late at night.