Why, thank you!
If I were in your situation and money were no object, I'd def. go custom-made or buy really high-quality vegan. I still own leather shoes - boots, mostly - that will probably last me the rest of my life so I'm going to wear them till they fall apart, but I'm not sure I would even buy leather second-hand. I do think it's better than buying leather 1st-hand, but it just feels like it's indirectly perpetuating the demand for leather. I would, however, take a hand-me-down, if that situation came up. I'd rather it go to me than into the trash or into someone's hands that didn't otherwise care.
I can afford a new pair of bespoke shoes and was going to go down that road, but now I've thought about it and decided that from impact perspective used leather is best. My thinking stems from the fact that most unsold used clothing items end up in a landfill (fact,) and thus will cause no upstream increase in demand (conjecture, but it's not like the guy selling shoes off his feet and will run out to buy a new pair).
If (and this is a big "if"), by buying a used pair of leather shoes you wouldn't increase the overall demand for leather shoes by one pair, then buying a used leather pair would cause less harm than buying a new vegan pair. (Because anything new we manufacture causes harm to animals somewhere along the line.)
There are few arguments pro and con it in my mind. On one hand, as you said, manufacturing is a bad thing and causes damage somewhere in some shape (pro "used leather"). On the other hand, whoever is selling those shoes will probably go and buy himself another pair (pro "new vegan"). On the third hand, supporting a vegan manufacturer is a good thing cause we can use more good vegan stuff (pro "new vegan"). On the fourth hand, most of the used clothing and shoes end up in the landfill which is bad (pro "used leather"). Finally, if you assume that I have a budget (I sort-of do, cause my first inclination was to go bespoke), spend say 100 out of 1000 and give the rest to a good cause (pro "used leather").
In short, I don't think there's a clear cut "right" answer for you. Do what you think is best.
Actually, I think from the pure quantitative perspective (least impact) there is a "right" answer. We just don't have all of the information to make that choice. In any case, I was leaning towards new vegan early this morning, but after some consideration decided to go with used leather.
PS. Someone should build a website that would go through impact choices like that so I don't have to think - thinking is expensive
PPS. To be honest, best thing would be to buy nothing, but sadly that's not going to work considering the industry I am in
