Dear Taraxaku Hikari,
Thank you for contacting us!
Please inform that Japanese passport holders are exempted 15 days from Vietnam visa. Thus, if you will stay in the country less than this duration, no visa is required. In case you want to stay longer than this duration, you have to apply for visa for Vietnam in advance. At present, Vietnam visa on arrival online is the best choice for Japanese who enter Vietnam by air and Vietnam visa at embasy is the best option for land and sea entries.