"Virgin" can be used figuratively, to mean that a person has never tried something before (and will probably experience it now). I suppose the phrase "vegan virgin" means the person is trying a vegan diet (no animal products at all) for the first time.

vegan = vegetarian, someone who does not eat meat virgin = a girl that has never had sex before, with her virginity vegan virgin = Vegan Virgin Valentine (2004) is a young adult novel by Carolyn Mackler. The book has been banned in some locations because of "inappropriate language".