It's a difficult one this for us to be honest as we have multiple reasons for being vegan. Animal welfare (or lack of !!) is the main one, but environmental reasons are also always in our thoughts when buying of food and products. Vegan shoes, and other plastic type leather replacements are very poor for the environment, they don't degrade, and they use oil are just 2 reasons we struggle. For these reasons I would buy second hand leather products; environmentally this is very sound, and ethically if an animal has died one can argue that we should use it for as much and as long as possible. I have an old leather belt I've been wearing now for 30 years, I would't stop using it now because we're vegan, that would be just wasteful in my opinion, and being a natural product well-cared for leather will last for years and years. Plastic belts when used daily fall apart quickly and soon end up in landfill.
I don't have an answer to be honest, daughter bought vegan Dr martens at christmas and I have some issues with them not being environmentally sound, but leather means the death of an animal. I guess it's whichever does less harm overall, but working that out isn't quite as straightforward as one twould hope!
This argument also holds for dairy replacement products, most of which rely on palm oil, a product we boycott due to it's harm to the habitat of the Orang Utans. Boycott dairy to save the cows suffering and then eat palm oil and inflict suffering on Orang's. Not a great choice to be fair!