Vegan leather is not as durable as a normal plastic made for covers. It's generally made of Mycelium (mylo) the specific Xiaomi vegan leather is made in part of apple peel.
The term Vegan leather is reserved for "sustainable" materials, which means it can be made of reused plastic. But that's still not as strong as the high end plastic materials that was originally used for premium phones.
If I knew for sure the vegan option for the phone was made of high quality PU, I would have chosen that.