Vietnam Visa on arrival is the type of visa picked up at Vietnam airports.
Visa on arrival (VOA) is the most comfortable way to obtain Vietnam visa without wasting time. When applying for Vietnam visa upon arrival, your visa approval letter will be issued by Vietnam Immigration Department. So it's a legitimate document and fully supported by Vietnamese Government.
With Visa on arrival, tourists will get their Vietnam visa stamped at Vietnam Airports so it's applicable to air travelers only. However, you are free to exit at any ports.
For this type of visa, you need to submit Vietnam approval letter together with your passport (at least 6 months validity), two passport-sized photos, entry and exit form and stamping fee to Immigration Officers at the airport, the visa will be stamped onto your passport.