Not a complete answer, but an usual group, in that is one not often associated with vegetarianism, were Roman gladiators. They ate a diet primarily plant based, but also included a tremendous amount of barley, to the extent that they were referred at time as hordearii or "barleymen". Though they did consume a sort of calcium supplement in the form a broth made of bone ash to make up from the deficit of a diet based mainly on legumes, grain and vegetables, and they ate A LOT. So much so that gladiators were quite chonky. Think the silver masked gladiator in the film Gladiator. The reason for this seemingly out of place diet in what is stereo-typically thought of as an ultra-macho red meat-eaters profession is quite ingenious. A lot of gladiators were not there through choice, often slaves. Gladiators as such are a commodity like a racehorse - if your horse is injured and not racing, it's costing the owner money and not bringing any revenue back in. It was in the best interest of a gladiator owner not to see them suffer debilitating injury. A wound to a muscle group causes a lot of damage, and will also quickly disable the fighter through damage and blood loss. Cuts to fat, however, aren't nearly as damaging to the vital muscle structures. Plus (and you really need to be in the Roman mindset to appreciate this) they bleed a lot, but superficially, and that looks great to the roaring crowd in the arena.

So you have an ultra violent, often coerced group of near celebrity level athletes in classical Rome, who deliberately eat an essentially vegetarian diet with supplements for the explicit reason of gore-factor and martial endurance. I would say that isn't quite a feeble link in the historical chain of vegetarianism!

https://archive.archaeology.org/0811/abstracts/gladiator.html for more on the diet of gladiators.