Vinegar will rust the surface of bare metal if it isn't neutralized after use, it's acidic.
I use it to strip rust out of the inside of old rusty motorcycle tanks, I fill the tank and leave it soaking. Every week I drain and strain the vinegar and refill till the tank is rust free, once rust free you dump the vinegar and flush the tank with fresh water. After the flush you neutralize the acid base with warm water and baking soda which is alkali, if you aren't quick enough with the baking soda the flash rust is immediate. Once neutralized you rinse with methal hydrate to expell any moisture and coat the inside with two stroke oil or fill with gas and use.