Vinegar lowers the freezing point of water from 32 degrees Fahrenheit down to 28 degrees Fahrenheit. When vinegar is added to water, it prevents water from freezing at its normal temperature and keeps it in liquid form.
Vinegar is a mixture of water and a chemical called "acetic acid." The acetic acid molecules form hydrogen bonds with the water molecules, gathering several water molecules around each individual acetic acid molecule. This bond holds the water molecules, preventing them from evaporating. When the molecules can't evaporate, this reduces the vapor pressure. Thus, one of the two forces is no longer acting as quickly on the water, and it takes more external pressure to freeze the water.