A generalized thermodynamic framework is provided for vapor-liquid equilibrium (VLE) in the presence of external fields and is utilized to develop a semi-empirical model for VLE within porous materials that accounts for the effect of the long-range surface forces. The alteration of VLE of binary mixtures in capillary porous plates is explained on the basis of the effect of long-range surface forces exerted by the solid on the liquid, which alter the intermolecular interactions amongst the mixture components as well as their vapor pressure. The excess Gibbs energy, activity coefficients, and relative volatility are studied in the presence of external fields in relation to those in their absence. The VLE of cyclohexane-ethanol (components of different polarity) system at 25 degrees C, and acetone-ethanol (similar polarity) system at 32 degrees C are predicted using an approximate approach developed in this study and compared with experimental results. It is found that the results obtained by the approximate approach are in agreement with the experimental ones.