Is vegetarianism compulsory/obligatory in Nicherin Buddhism? I recently visited Japan and went to see a Nicherin Buddhist monastery and temple in Tokyo. There they were preaching that Buddhism is not just for the Japanese or Chinese or some particular race, it is for the entire humanity. The chief monk their would barely speak English but he said he is a follower of Buddha, Nicherin and Tanaka Chigaku. Then he invited me to his home and his wife served fish sushi and chicken meat balls for both of us! I ask, 'You are a Buddhist and an ordained monk, do you eat meat?' He replied that even Buddha and his followers ate meat!
There are three lineages of pratimoksha vows and none of them prohibits the eating of meat. Anyway, the transmission of the pratimoksha was lost in Japan, as attests the fact that the "monk" is married and lives with his wife.

He surely has the bodhisattva vows, however. There are also a number of lineages of bodhisattva vows. Some prohibit the eating of meat and some do not. For instance, in the Brahma Net Sutra, it says:


  A disciple of the Buddha must not deliberately  eat meat. He should
  not eat the flesh of any sentient being. The meat-eater forfeits the
  seed  of Great Compassion, severs the seed of the Buddha Nature and
  causes [animals and transc endental] beings to avoid him.


The bodhisattva vows that I have (in Tibetan Buddhism) do not prohibit the eating of meat. Neither does it advise to restrain from eating meat.

Most Chinese, Japanese, Korean, Vietnamese have the bodhisattva vows from a lineage rooted in the Brahma Net Sutra, and therefore are encouraged to restrain from eating meat.

Even then, practitioners do not adhere that strictly to the bodhisattva vows.