Yes, Vietnam VOA is a legitimate process and your visa, which is authority issued by Vietnam Immigration Department has the same validity with the visa you get from the Vietnamese embassy abroad. You can see "Visa on Arrival" is stated well in the Immigration Laws of Vietnam (link). You can check out the Ordinance on entry, exit and residence of foreigners in Vietnam: (01) Government’s Decree No. 21/2001/ND-CP. (2) Joint Circular No. 04/2002/TTLT/BCA-BNG. A VOA also has been mentioned in various news outlets, some of which include The New York Times, USA Today, Trip Advisor, and CNN.