A Catholic would have serious issues with such a practice or agenda.This. Put much better than I could.
WHY are you abstaining from any product that comes from animals? Because they "animals are people too"? Because monkeys evolved into man, and so for all we know dolphins and other animals might be intelligent life? Because God didn't give animals to us for our use? Because only cavemen (like the Patriarchs, prophets, Apostles, saints, etc.) would be so primitive as to eat meat? Because there isn't a God in the first place? Because God doesn't know what's good for our health?
Think about it.
Also, whatever health benefits vegetarianism provides, there is NO health benefit to not using "animal products" like leather. The only reason to abstain from leather is due to pagan, new-age, or other non-Catholic beliefs.
Let's put it this way: abstaining from all animals (and animal products) in a vegan manner suggests that "God made a mistake" -- a blasphemy that any Catholic should hold in horror.