Desiccants are an important part of dehumidifier systems, contributing to absorbing the humidity of the air. The vapor pressure is one of the most important properties in the dehumidification process. For this reason, the application of the Cisternas–Lam model can overcome difficulties such as the lack of experimental vapor pressure measurements. In this work, this model is applied to aqueous solution of several desiccants such as CH3COONa, CH3COOK, NaClO4, and KClO4, including ionic liquids such as [EMIM][OMs], [EMIM][OAc], [EMIM][OTf], [E3MPy][OMs], [EMIM]Cl, [BMIM]Br, [BMIM]Cl, [BMIM]I, [EMIM][ES], [EMIM][MS], [BMIM][DBP], [EMIM][DEP], [MMIM][DMP], [EMIM][DMP], [EEMI][DEP], [EMIM][Tf2N], [EMIM][DCA], [DMIM][BF4], and [MMIM][MS], estimating their K value in binary aqueous systems. In addition, the prediction capacity of the model is tested in interpolation and extrapolation examples and shows good results. Finally, this model is used to predict the vapor pressures of aqueous ternary systems that contain ionic liquids. The results showed that the Cisternas-Lam model is appropriate to correlate and predict vapor pressure in the systems studied.