A local guide would be invaluable to recommend vegetarians options. Also, be very explicit if eggs or milk are acceptable, because there's very few strict vegetarians in Japan. Even monks and nuns who practice Buddhist vegetarianism consume milk.
IIUC, the original Buddhist teaching is that getting involved in taking life smears your karma, not necessarily that consuming the results will. Confucianism influenced branches tends to take the latter interpretation so that's what a lot of Asian(Japanese or Chinese) guys would say, but hardcore Indian style counterintuitively is that everything already on the platter is lost cause and even wasteful to refuse, given one does not actively seek it or support it.
My highschool teacher had an anecdote on this. There was a small, yearly post-event party which a Buddhist guru is invited to join. One time there were couple chicken wings for each. They feared the guru would have some words on consuming meat, or others next to him has to take the extra, but he saw no problems with it since it was offered and presented to him passively, and just stated he has to reasonably clean the bones and that that isn't a fast process. From next time on the party had become strictly boneless, but not vegetarian.
