Blot wet nail polish on a leather jacket with a paper towel. Touch the towel to the polish and pull away without smearing it, turning the towel to a clean section and repeating until you've removed as much as possible.
Dip the tip of a cotton swab into vinegar and dab it on the remaining nail polish. Allow the vinegar to sit for five minutes to lightly soften the spot. Do this whether the nail polish stain is new or old.
Scrape as much of the nail polish as you can off of the leather jacket with a putty knife. Be very gentle, as the knife can leave scratches on the leather.
Dab any remaining nail polish with a fine-tipped paint brush dipped in nonacetone nail polish remover. Blot with a paper towel. Repeat, taking care not to get the nail polish remover on the leather, until the polish is gone.
Sand away any remaining residue with fine-grit sandpaper. If necessary, touch up the spot with leather touch-up paint in your jacket's color.