To enter a foreign country, the first thing is to apply for a visa of that country (except in the case of visa exemption). Visa on arrival is a different way to obtain a visa for Vietnam other than getting a visa at the Vietnam Embassy.
For this type of visa, you will need to fill in the online application for a visa approval letter through a visa service company or organization, then you will receive a visa approval letter through email. After coming to Vietnam, you need to submit to the immigration officer at the airport documents including approval letter, original passport (valid for at least 6 months), 2 passport photos, and Vietnamese visa application form and stamping fee to Immigration Officers at the airport.
There is a basic difference between visa on arrival and visa at Vietnamese embassy. With visa on arrival, the Vietnam Immigration Department will issue a visa approval letter with your name on it stating that you are allowed to enter Vietnam on a specified date (according to the application form when you apply online). Then, you will have the visa stamped onto your passport at the arrival airport in Vietnam.
Visa on Arrival service is the best choice for those who are far away from the Embassy or the emergency does not have regular visa time.