@thisismissem @anarchopunk_girl
(Apologies for the long post but this is an interesting point.)
Since you asked specifically about the medieval era (that is, in Europe between roughly 1000 and 1500, which is what we normally mean when we say "the middle ages") diet was a complex thing for several reasons, made more difficult to study by the limited number of written sources about the lives of peasants.
Firstly, meat was a status food. In some parts of Europe, records use people's poor diet as a byword for how poor they are, and this sometimes references meat. For example, there are records from Scotland saying things like "this widow was so poor that her only meat was an occasional bit of bacon in her stew." On the opposite end of the spectrum, aristocrats often suffered dietary medical problems which we now recognise as being due to a diet with too much meat and too few vegetables. Chaucer uses "meat" as a synonym for "aristocrat food", and there are references from Poland where aristocrats react with alarm to lowborn-but-well-off people eating more meat than their station in society dictated.
Secondary, Christianity at the time had dietary laws about which foods one could eat when. These weren't always obeyed, or even enforced, and we know this from the sheer number of records from church people getting angry and demanding that "from now on, the dietary laws must be obeyed." However, even if not always obeyed, this gave a social context to abstaining from meat: it was seen as a godly thing to do, admirable if perhaps unworldly.
These two things combine in many sources, because medieval Christians (and many Christians today) viewed humility as a good thing, and this humility often meant varying the amount of meat in your diet. Many pilgrims abstained from meat during their pilgrimages, and some holy figures like St Francis ate what we would interpret as vegetarian or vegan diets.
How did people view them? Well, as literal holy people, but also as kinda sanctimonious. Literally "holier than thou", if you will. Nobody was going to call St Francis a prick in writing, but many people called pilgrims pricks, and in some areas like Germany or Spain we have some sources referring to them as what we'd now call "poverty tourists."
However, this sort of poverty tourism was extremely popular in the medieval world: people liked festivals where they acted as if they were temporarily higher or lower in the social hierarchy, whether that was being "king for a day" or living like a pauper to ask repentance of sins. Doing it long-term or permanently was more unusual.
So, did principled vegans exist at the time? Yes, absolutely. St Francis talked about not eating animals because it was cruel to the animals, and he was an extremely popular figure. But it was understood in a different context from today: radical Franciscans were seen as revolutionaries (and many saw themselves as such) because their insistence on meat-free diets was interpreted as a threat to social hierarchy, rather than as an animal-rights or sustainability issue.
This is a subplot in Umberto Eco's novel Name of the Rose, interestingly.
(I won't talk about monks here because they are a really interesting part of this topic but not one that can be covered in brief.)