Bastet, the stuff you say is cute and all, but it's no evidence that vegetarianism was present in medieval times in any notable degree.
Agriculture allowed people to feed more folks. Folks multiplied. Obvious reasoning shows the diet was then consisting more on plants, not because people were suddenly vegetarians, but because there was more plants than meat. Anyway, as far as Europe goes, vegetarianism could be first associated with the classical period (Greece, Rome, and all that stuff), when philosophical thought began condemning violence towards animals. It began disappearing towards the end of Ancient period and was finally revived to some point during the Renaissance. All the same, until 20th century it remained at low levels.
But that's Earth.
As far as Yliakum goes, unless I see people walking around spreading idea of non-violence towards Trepors and Tefusangs (and people actually avoiding fights with the wild beasts), I see absolutely no basis for vegetarianism in PS.
The way it is now, Tefusangs are commonly hunted down, kikiris bred for food, innocent mounts sometimes suffer when criminals are cast into Azure Sun, and finally stronger animals are used to power up machinery.
Morality towards primitive animals is completely alien to a typical dweller of Yliakum.