I think I understand, but I want to make sure. The vapor pressure of a substance is the pressure of the substance evaporating/sublimating at a given temperature and can be calculated using the Antoine Equation. This must be measured at the interface of the substance and atmosphere, or if the system is at equilibrium, it could be measured anywhere in the system. The saturation pressure assumes that the substance has fully vaporized to its equilibrium point. It could be measured at any gaseous part of the system. They are almost the same, right? The difference is just whether the system is at equilibrium?