Visa is NOT required for citizens of Japan to stay in Vietnam for less than 15 days. If you intend to stay in Vietnam for longer than 15 days, then you have to apply for a visa and may have 2 options: Visit Vietnam embassy in Japan (Tokyo) or apply for a visa on arrival to Vietnam from Japan online. Japanese citizens may apply for both tourist and business visa to Vietnam for the stay of 1 to 6 months from Japan. Please check this link for visa fee for Japanese citizens.

All the information for getting a visa from Vietnam embassy in Japan can be found here: http://www.mofa.gov.vn/vnemb.jp/