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?
I'm not sure. I'd guess the Romans were tied to the Greeks, most likely. Vegetarianism didn't come back into widespread practice until the 19th & 20th centuries, though.