As far as I know Japan was not involved in Vietnam in any way other than providing logistics through U.S. military bases, mainly through Okinawa which was administrated by the U.S. until 1972 (correct me if I'm wrong).
Japan had its own anti-war movement with violent student demonstrations from 1968 on. A few of the more radical students later formed Japan's notorious terrorist cell, the Japan Red Army (Nippon Sekigun) that engaged in a few bloody attacks, but I'm getting off topic.
I forgot the title, but I've read a book written by a Japanese journalist who covered the Vietnam war...
On a side-note: you should check Korea's involvement in Vietnam. They did the "dirty work". Until today Vietnamese don't look too favorably at Koreans.