No, you cannot.
In order to board the plane, you must show the approval letter to the airlines. This letter is also compulsory in order for you to get the visa at the Vietnam airport.
Therefore, we recommend that you leave plenty of time to apply for the visa to avoid having to delay or change your flight.
Citizens of ALL COUNTRIES in the worlds are eligible to apply for Visa On Arrival Vietnam. Unlike Visa On Arrival of some countries, Visa On Arrival Vietnam required applicant obtain a visa approval letter (also called pre-approval letter) before flying to Vietnam to pick up their visa upon arrival at the airports.
Approval letter (also called pre-approval letter) for visa on arrival is a document issued by the Immigration Department of Vietnam (Vietnam Ministry of Public Security). It functions as an official permission allowing its owner(s) to board the flight to Vietnam and get visa stamped into the passport upon arrival at international airports.
The landing visa counter at international airports of Vietnam open 24/7 for your visa on arrival. Please remember to print a copy of your approval letter and bring it with you for your trip to Vietnam.