A contrasting perspective from European market knowledge:
Cow leather in Europe comes almost exclusively from adult cattle. Calf leather is usually labelled as such and is more expensive, so I would say it needs to be treated as a separate product, which I won't really get into here.
While leather does form a significant part of the value of a slaughtered animal, it is, at least in Europe, basically never the case that a cow is killed for its leather. Once a cow is mature, the value of its leather will be proportial to its size, and its quality doesn't really deteriorate with age over the span of the dairy or meat industry lifespans.
Leather in its raw form has 7 layers, the top ones being easier to work than the lower ones because they require less polishing. Most production uses only the top layer and the rest is completely thrown away.
As a result, there are very few leather farms. Cows are grown for their meat or milk, not for their leather.
At this point, it becomes a question of why you are Vegan. Some vegans are ok with eating roadkill, some not. Economically you are very slightly supporting the dairy and meat indsutries but you're not really increasing the demand, which is the main economic incentive to boycott industries.