Why do the vinegar and oil in a salad dressing separate into layers if left standing for too long?

 In the case of oil and vinegar, the vinegar is polar and more dense than the oil, so it settles on the bottom of the container. The oil is nonpolar and less dense, so it doesn't dissolve in the vinegar, and it floats on top.

Because they  have different densities which cause them to separate .