Oil is a nonpolar substance and water is a polar molecule - this is a key reason why water and oil do not mix.
Water molecules have positive and negative ends. The charged areas of water molecules help water dissolve polar solutes. Some substances are nonpolar. A nonpolarmaterial does not have positive and negative areas.
Nonpolar substances are not attracted to polar substances, including water. Most nonpolar substances do not dissolve in water.
Because water molecules are electrically charged, they get attracted to other water molecules and exclude the oil molecules. This eventually causes the oil molecules, or lipids, to clump together.
Acetic acid, the active component of vinegar, is easily dissolved in water because it is able to form hydrogen bonds with the water, while nonpolar oil molecules do not. That is why you have to shake a vinegar-and-oil dressing to mix it before using it.
To learn more, visit the following sites:
HumanTouchofChemistry.com
ScienceFriday.com