I don't think many wore leather vests in the old west. Not that they didn't exist but fabric was more common. Many pants didn't have pockets so vests often had four pockets to carry things needed like a watch, knife, tobacco, coins, notebooks, pencils, etc. The movies had one thing correct: almost all men wore a vest. If you cruise through some of the historic clothing sites, you'll get an idea of what was done with them. On the trail, I would suspect that vests would be of a more durable cloth with more pockets where a townie might have one of silk or other light fabric and maybe only a couple pockets. Just how common leather might be would take some researching.