There is a wide range of websites that offer Vietnam visa on arrival service. We advise you do one of the following:
Check your email to see if there is any email relating to visa to Vietnam. If yes, contact them via the email that you receive or google for their website to contact them by other ways.
If you already make the payment, you are highly recommended to contact your bank to see which company name was on the charge/statement.
Use your web browser history (Firefox, Chrome, Safari, etc.) to track back the websites about Vietnam visa that you visited before.