Hi!  I've gotten into veganism recently, but it seems like veganism isn't really an old thing.  It must've not existed 100 years ago.
You might be shocked to find out that it has roots in the civilizations of ancient India and ancient Greece. 
Wow definitely color me shocked.  With india is veganism related to religion since they don't like eating mutton and cows?
Yeah, in both cases the diet was very closely connected to the idea of nonviolence towards animals. 
Outside of India has veganism only cropped up in modern times, or perchance was it a fad in victorian England?
After the Roman Empire was Christianized in the 4-6th centuries, vegetarianism almost disappeared completely from Europe. It reemerged somewhat during the Renaissance. 
Do historians know why the christianization of Roman empire eliminated veganism and why Veganism was in the roman empire before hand?  Is it cause the Romans borrowed from the greeks?
banned the consumption of meat for ascetic reasons, but none of them abstained from the consumption of fish; these monks were not vegetarians, but some were pescetarians. Vegetarianism was to reemerge somewhat in Europe during the Renaissance and became a more widespread practice during the 19th and 20th centuries. The figures for the percentage of the Western world which is vegetarian varies between 0.5% and 4% per Mintel data in September 2006. 
Ancient.
Ancient Indian subcontinent.
Ancient Indian subcontinent Early Jainism and Buddhism.
Jain and Buddhist sources show
and Brooklyn.
Activism.
Moskowitz is a vocal opponent of "humane meat", promoting animal rights through what she calls "Culinary Activism" or "Baketivism":
I think that activism isn't what you decide to do but how it affects people. So if someone says, I'm going to become an activist! I'm going to stand on a street corner and preach about veganism! And then they go ahead and do that but no one listens and no one becomes vegan, then is that