If you need to stay in Vietnam for more than 24 hours during your layover in the country, then you will have to apply for a transit visa. What’s good about getting a visa is that you are free to get out of the country to explore. You will not be stuck at the airport while waiting for your next flight.
Having a Vietnam transit visa is also necessary if you need to travel within the country from your point of arrival going to another place in the country for your departure. For instance, you are arriving at Hanoi airport but you need to get out of the country from the seaport of Hai Phong. In this case, you need to be able to present a visa at the Vietnam Immigration in Hanoi for you to be allowed to enter the country and travel to the seaport of Hai Phong where you will be taking a boat to exit the country.
Transit visa is a short-period, temporary visa needed in some countries to stopover for a minimum of 24 hours in the country. If you are traveller who is about to stopover in Vietnam to get your destination flight, then you may need a transit visa.