This is probably the easiest way to obtain your visa without having to chase down Vietnam embassies, consulates and the like prior to your trip. It is a valid alternative when applying for both tourist and business visa. The Entry visa will be stamped on your passport as you pass through Vietnam Immigration Checkpoint. Though valid for thirty- ninety days, it can be extended once you are here in Vietnam. To get a Visa upon Arrival, you need the support from the travel Companies. After receiving the Visa Approval Letter for you from the Vietnam Immigration Department, they will send you a copy by fax or email. Copies of the same document will be forwarded on your behalf to Vietnam Immigration Checkpoints at International Airports only. When you arrive in Vietnam, the Immigration officers will have those documents on hand and will be able to issue your Entry Visa expediently./.
Here is sample of the Visa approval letter.