Yes, it is. Visa on arrival is the other way to get a visa to Vietnam, which is said to be the easiest and simplest one. It is legitimated by Vietnam Government, and is more widely used today thanks to its favorability.
The condition for visa on arrival to be legitimate (or for visa applicants to be allowed to get visa stamped at arrival airport) is that the visa applicants have “visa approval letter” in hand to show at the arrival airport in Vietnam.