Vinegar is dilute acetic acid, and on feet it does two genuinely useful things. Neither is magic, and both are cheap.
It changes the skin's pH
The fungi behind athlete's foot and nail infection need a fairly narrow pH range to thrive. Acetic acid drops the pH of the skin surface and they struggle. That is the entire mechanism, and it is a good one — simple, well understood, and hard to get wrong.
One part vinegar to two parts warm water. Twenty minutes daily. Then dry completely, every crevice, especially between the toes — because leaving them damp undoes the whole exercise.
It softens hard skin
The same acid breaks down the keratin bonds in thickened skin, which makes vinegar excellent before working on a callus or corn. Soak twenty minutes in the same one-to-two solution, then file gently with a pumice stone. Never cut.
Apple cider or white?
Both work. Apple cider vinegar is gentler on the skin and smells better; plain white vinegar is cheaper and slightly stronger. There is no meaningful advantage to expensive raw unfiltered varieties for external use.
What it will not do
It will not cure an established nail infection on its own. Vinegar struggles to penetrate the nail plate, so it works best alongside something that does — tea tree or oregano oil applied directly.
It will not remove a callus in one session, dissolve a corn, or fix the pressure that caused either.
Drinking it does nothing for your feet.
The cautions people skip
- Dilute it. Neat vinegar held against skin for twenty minutes will irritate and can burn.
- It stings badly on cracked skin. Close the cracks first.
- Twenty minutes is a limit — prolonged soaking waterlogs skin.
- If you have diabetes, do not use acid soaks without medical advice.
Getting the most from it
Alternate a vinegar soak with a tea tree soak rather than combining them, so the skin gets variety without extra irritation. Treat shoes at the same time — a fungal infection reinfects from footwear endlessly if you only treat the foot.
Full entry on vinegar; see also athlete's foot and toenail fungus.
Treating the shoes as well
A vinegar soak treats your skin. It does nothing at all to the shoes, and shoes are where a fungal infection waits out your treatment before reinfecting you.
Wipe the inside of each shoe with a cloth dampened in a one-to-one vinegar and water solution, paying attention to the toe box and the seams. Then let them dry completely — a full day, out of the shoe cupboard, with the insoles removed and dried separately. Damp shoes treated with vinegar are simply damp shoes.
Do the same to the bathroom floor and the shower tray if the infection has been recurring, and wash bath mats hot rather than warm.
One caution on materials: vinegar can dull or mark some leathers and adhesives. Test an inconspicuous spot first, and use a barely damp cloth rather than soaking the lining. For shoes you cannot treat this way, a fortnight in a sealed bag with a generous amount of cornstarch draws out the moisture the fungus depends on.