I think the water will just drive the mold deeper into the leather, household vinegar already has a lot of water in it and will have, imo. the effect of feeding the mold.
First of all don't do anything until you have satisfied yourself you aren't making it worse. You have access to the entire internet of information, about half of which is true, so do your research.
I won't say that I know much about mold on leather, just that I lost some leather products to mold while trying stuff like vinegar. i also managed to salvage some.
A professional mold remover in buildings told me that all they use is concentrated Hydrogen Peroxide, too dangerous for consumer use, he said.
I did find strong sunlight reduced/killed mold and that a laundry Borax solution killed and seemed to prevent mold. But leather is hard to say what works best.
I would take the leather outside first, because even brushing the mold inside will spread the spores throughout your house. Once outside, I would brush off all that I could dry, then I would sponge it with undiluted 3% hydrogen peroxide, being careful not to soak the leather. I might repeat that a couple times using a clean sponge/rag each time and fresh solution. Then after drying, work the leather with Fiebing's Saddle Soap or Lexol as needed. I also sometimes use pure glycerin and water 50/50 to soften leather depending on it's condition.
But, this is just what I would do. If anyone comes along has better ideas or more experience, I'll learn from them.
