There’s probably a good chemical reason why vinegar and salt don’t like each other, but I don’t know what it is. I was probably too busy making ink pellets in the back row when that relationship was explained in Chemistry class.
The enmity is very useful, though. For when you’ve been out in the rain a lot (as happens frequently at this time of year) or been sweating rather profusely (as happens in the other half of the year), vinegar is the best way to tackle the salt stains that can result.
As the leather of your shoes absorbs water, salt gathers at the high-water mark – usually about half way up the vamp and an inch high around either side. If this is allowed to dry and not tackled quickly, it can leave a permanent ridge on the leather. Like scum left by a retreating tide.
When the shoes are completely dry, make up a mixture of 50:50 water and vinegar. Dip the cloth in the mixture and rub it over the salt stains. The leather will noticeably darken as it absorbs this new liquid, but don’t worry about that. Concentrate instead on the ridge of salt that stands away from the shoe like dirt. You want to keep rubbing the solution onto this ridge, and reapplying, until it dissolves and the leather is smooth.