Visa on arrival works if you arrive at any of the four international airports: Noi Bai airport (Ha Noi city), Tan Son Nhat airport (Ho Chi Minh City city), Cam Ranh airport (Nha Trang city) and Da Nang airport (Da Nang city).
There is no problem with switching between these four airports.
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.
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.
In most cases, a passport scan is not required. However, sometimes the Immigration Officers need to verify the information you provide, you are required to send them a scan copy of your passport.