We arrange visa upon arrival at the airport for those entering Vietnam by air and wish to get visa at the airport. Therefore your visa must be picked up at the airport. You should contact Vietnamese embassy to get visa if you do not enter Vietnam by air.
You must have pre-approved letter from Vietnam Immigration Department that allow you to pick up your visa upon arrival to Vietnam at the airport. Without this letter you can not board the airplane and get visa when you arrive.
When you arrive at the airport of Vietnam, you will see "LANDING VISA" counter before you check-in, come to the "Landing visa" counter, show the pre-approved letter for checking on the system then wait for your visa stamped on your passport.