If You are applying for Visa to Vietnam, You see two options to apply for a single entry visa or a multiple entry visa. Although both are valid for a fixed period of time, they are different from each other in terms of number of entries during the visa validity.
If you hold a single entry visa, you are allowed to enter and exit Vietnam one time only, during the visa validity. Once you have left Vietnam, even before the granted expiry date, your visa will automatically become invalid and if you want to come back Vietnam, you need to apply for a new visa. This kind of visa is also called “a visa with single use”