Answer
Firstly, We are guarantee that Vietnam visa on arrival is not scam. As you can see, the approval letter for your visa on arrival is approved by Vietnam Immigration Department (Vietnam government). We guarantee that the approval letter is official from Vietnam Immigration Department and you can get visa on arrival without problem when you get visa on arrival at Vietnam airport. You will see landing visa office in front of immigration line at the airport of Vietnam. I can tell you that we will going jail if this approval letter is fake.
Dont worry, we have 10 years experience in arrange visa on arrival, we cannot live till now to providing Vietnam visa on arrival service if it is not true. I dont know why the embassy saying that but I can tell visa on arrival Vietnam is legitimated.
Please see official document from Vietnam government about Vietnam visa on arrival if you doubt about my words to cheat you.
Ordinance On Entry, Exit And Residence Of Foreigners In Vietnam. Click here
Decree No. 21/2001/ND-CP of May 28, 2001 Detailing the Implementation of the Ordinance on Entry, Exit and Residence of Foreigners in Vietnam. Click here
Joint Circular No. 04/2002/TTLT/BCA-BNG of January 29, 2002 Guiding the Implementation of the Decree No. 21/2001/ND-CP of May 28, 2001 Detailing the Implementation of the Ordinance on Entry, Exit and Residence of Foreigners in Vietnam. Click here
Circular 190/2012/TT-BTC - Amending and supplementing the circular No.66/2009/TT-BTC, of march 30, 2009 of the ministry of finance stipulating the regime on collection, remittance, management and use of fees on passports, visas and papers on entry, exit, transit and residence in Vietnam. Click here