Forget all the stories about getting sick on holiday because of the strange food. That sort of thing has never bothered seasoned travellers. In some countries, there is far too much bureaucracy when it comes to environmental health. This in no way suggests that places that are unhygienic should be allowed to serve food to the public but the vast majority of the places that have has problems have had them because of their ingredients not being fresh. That will never ever be a problem in the streets of Hanoi. Street vendors serve locals breakfast on the way to work, lunches to all that require them and plenty of meals later in the day. Their turnover guarantees that everything they use is fresh; it will have been bought that day at the market, and sometimes street vendors will visit the market more than once.
If you want to only eat in restaurants that is your choice but part of the adventure of all holiday in Vietnam is to mix with the locals and their daily lives. A street food tour of Hanoi is something you should seriously consider; your travel operator can include one in your Vietnam travel package during your time in the City. Many people start their travels in Vietnamby flying into Hanoi, picking up their Vietnam visa on arrival travelling if they require one into the City.
You will have a choice about the time of day you want to take a tour. Street food as already mentioned is available so the locals can have breakfast on the way to work. It’s a busy time and maybe you prefer for the crowds to die down before you take to the roads. You can do a tour by motor or scooter as well as merely walking around the Old Quarter, a very popular option.
You will have a guide who can speak English to explain what each of the dishes is and the cost of all the food will be included in the price you pay locally for a tour, including collection from your hotel and return. Alternatively, if you have requested a tour as part of your overall Vietnam tour package you will already have paid.
There are several dishes that you will have the chance to try. Pho is noddle soup with usually pork added but sometimes other meat or fish. There will be vegetables, perhaps rice pancakes or spring rolls; the choice is yours. The locals eat similar meals at every sitting as opposed to in many western countries when only certain things will be offered for breakfast and dinner will be something completely different.
Eating street food will be something new to all but the seasoned traveller. Go ahead and try it, especially as an introduction to the Country. It will be cheap, full of flavour and nutrition. You will certainly opt to do something similar later in the holiday on days when restaurant meals have not been pre-booked.