Normally, 1 photo is required for Vietnam Visa on arrival. This photo will be attached on the form NA1 (photo box in the first page of the form NA1 ). But you are recommended to prepare 1 more photo. In case the officer requests you can submit to them immediately.
The photo could be in 3×4 cm, 4×5 cm, 5×5 cm or 4×6 cm in blue or white background.
Note: If you do not have time to prepare those photos before your departure, you can also get them taken at a Vietnamese airport. The extra fee will be charged.