It's a term I've heard in my country.
It's basically a metaphor for getting the same thing, but without involving real living beings...
I think it's largely true, as it makes it healthier.
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Might be lost in translation.
I get the meaning of "not the real deal". Like vegan meat. In that sense you might use it for hentai.
Making the step from not the real deal to also not being passive but an active medium is too much for my language sense.
Also, vegan porn does have a somewhat established meaning. A quite literal meaning. It is porn by, from, with or for vegans. In the most blatant sense, it involves certain vegetables.
There is vegan water. Not kidding. It is water bottles that promise not to use glue made from animals for the labels and not use gelatin for filtering the water. And there are bottled beverages that do use gelatine for filtering. That's actually involved for a lot of beverages, though it is less used in large scale productions.