Depend on the chosen processing time and the purpose of applicants when apply visa on arrival. The normal processing time is 2 working days. In urgent cases, you are able to have the visa approval letter within 4 – 8 hours. And in super urgent cases, your visa approval letter will be sent around 30 minutes – 4 hours.
In addition, the time to process for a visa upon arrival on weekends and holidays is more special.
Have a nice day!
Vietnam Immigration Team