After having kept jellyfish as pets (Atlantic bay nettles), I wouldn't really consider them to be vegetarian nor vegan. While similar to plants, seemed to have a greater sense of environmental awareness than my plants. Mine could sense light, have "off days", and interact with their environment. It's probably true that there's not much going on there due to the small amount of nerves that control everything, but even when mine would accidentally get caught on tank cleaning tools or get bumped around they'd react in a protective way and to me it's just similar enough to animalistic behavior that I'd not feel comfortable consuming them if I were vegan.