Apply for a Vietnam visa approval letter or invitation, which is a legal document which is issue by the Vietnam immigration department and through the help of this, you can be permitted to board the flight and get your visa stamped on your passport after the arrival in any of Vietnam’s international airport. If you’re flying into Vietnam it’s usually easiest and cheapest to get your visa approved through an online visa-service offered by us.

Source: http://www.vietnameseprivatetours.com/vietnamese-visa-approval-voucher