Why is vapor pressure independent of volume?
Vapor pressure is oppposite in direction from atmospheric pressure. Vapor pressure is the pressure exerted by a liquid back on the atomosphere. Vapor pressure depends on the nature of the liquid and the temperature.
An example is water's vapor pressure, which happens to be relatively low because of the hydrogen bonding between the water molecules. No matter what volume the water is, the vapor pressure of the water is the same as long as the temperature is not change.
Hope this helps!
Really good detailed explanation on this page http://www.chemteam.info/GasLaw/VaporPressure.html