Just for the record, Giorno Giovanna is canonically not a COMPLETE vegetarian. This isn’t even a case of it never being said one way or the other, either; I have definitive proof from the comic itself.
The most common scanlation freely made crap up (“oh lolz”), so just to be sure of accuracy, here’s the actual original character bio for Giorno — because Garrick loves ya, baby.
Now, I’m guessing the origin of the fanon is this bit here:
[Least favorite food: Bird meat, particularly duck]
But look carefully at the line above it, emphasis mine:
[Favorite foods: Chocolate, pudding, octopus salad]
Oh dang! Octopus is not a vegetable, to my knowledge. Apparently Giorno is less of a hippie than we thought.
"But Garrick!" you say. "The word ‘tako’ (octopus) there is in katakana, not kanji; could it be that Giorno’s third-favorite food is, in fact, taco salad?!”
That was actually my first thought as well, oddly enough! And it’s totally possible to have vegetarian taco salad; my brother-in-law eats it all the time. However, the Japanese term for taco is タコス [takosu], not タコ [tako]. In a culinary context, タコ pretty much always refers to octopus. What’s more, a cursory image search for タコのサラダ turned up a lot of bowls of tentacles, and no Tex-Mex to speak of (and taco salad doesn’t seem to be that well-known outside the US anyway). As far as I can tell, the item Giorno likes to eat is, indeed, salad with octopus.
Now, granted, there are in fact plenty of vegetarians who eat fish (do octopus count?), but at the very least, Giorno likes chowing down on some dead animals. Which makes a modicum of sense, as he does sometimes kill humans as well and doesn’t generally seem particularly bummed about doing so.
tl;dr Giorno eats our future gastropod overlords on salad, but some vegetarians IRL do that, so it doesn’t necessarily ruin all the fanfics.
In closing, here is Mista’s terrifying maw as he, by sheer coincidence, also talks about the relative merits of eating vegetables or meat:
(This image alarmed me while I was looking for the bio page, so now I’m going to inflict it on all of you as well.)