The photo requirement should be the same as the requirements for the Vietnam e-visa photo. It must show your face clearly and it should be recent. The background should be white and must not have any filter. It should clearly show your face to indicate that you are the visa applicant. Otherwise, the immigration officer might not process your Visa on Arrival.
Unlike the Vietnam e-visa photo that will be submitted online, the photo for Vietnam Visa on Arrival must be attached to the actual form that you will fill out as you arrive at the airport of Vietnam. Therefore, you should bring the printed photo and not the soft copy.
Here is the Vietnamese visa application that you are required to filled, signed and attached your photo on it. Vietnamese-visa-application-form (Form NA1)
