You have visa approval letter and some required documents, this is easy to find visa desk at international airport in Vietnam. Once you enter the airport and are on your way to the baggage reclaim keep an eye out for signs saying ‘Landing Visa’ or ‘Visa Upon Arrival’ office.
Depending on the amount of people arriving at the same time as you needing a visa on arrival, there may be a queue. As a rough guide it usually takes between 15 and 30 minutes to get through. To speed up the process just make sure that you have all the required documents ready and easily accessible.