When doing Visa On Arrival procedure at the airport, you need to prepare some required documents in advance:

• The approval letter (print it out)
• The visa application form. It would be better to complete it in advance by downloading this form via website.
• Your original passport: must be valid for at least 6 months from the day you enter Vietnam. If not, you can’t enter.
• The visa stamp fee (In cash of US dollars or VN Dong). The office doesn’t accept any other currency.
• 2 passport photos in the size of 4×6 (one is used for the visa application form, one is for backup case)

Therefore, it would be better to complete the visa application and stick your photo on it in advance. In case you forget to bring photos, there is also a photo booth at the airport. You can also take photo here. To be better, you should bring it.