http://www.epa.gov/ttn/chief/ap42/ch07/final/c07s01.pdf page 56
Under the ASTM Method D 323 (Reid vapour pressure), it is the absolute vapour pressure exerted by a liquid at 100°F. The higher this value, the more volatile the sample and the more readily it will...
They are the same when the air is saturated and the relative humidity is 100 percent.
Vapor pressure is the pressure produced when vapor particels above a liquid in a sealed container, collide with the container walls.
True Vapor Pressure is the pressure of the vapor in equilibrium with the liquid at 100 F (it is equal to the bubble point pressure at 100 F)
its just under 0.5 pounds.