Take note that the Vietnam e-Visa is valid at 28 points of entry in the country. These include border checkpoints and international airports. You can refer to the official website of Vietnam e-Visa for a full list of airports that the visa is accepted. List-of-port that accept for e-visa.
So once you have your visa, carry a printed copy with you on your flight. This must be presented upon check-in and of course, when passing through immigration in Vietnam.
