I have been pondering this. Most non-vegans do wear leather, figuring it is a by-product of the meat production process; they are not themselves killing any cows. But when you buy leather, you make animal slaughter more lucrative. This suggests that you will get more of it. It may push down the cost of meat, or encourage ranchers to put marginal pasture into grazing, but either way, wearing leather probably means more dead pigs and cows. Even restricting your purchase to vintage items ultimately probably contributes to the death of extra animals, since vintage shoes are substitutes for new ones, and if you buy that pair of pumps, someone else will splurge on a new pair instead.