Volume swelling and plasticization of glassy polymers as induced by gas and vapor sorption is of relevance in a number of industrial processes and applications, from foaming to coating and drug delivery. The thermodynamic analysis of solute sorption in glassy polymers is made complex by the nonequilibirum nature of the polymer-solute system, which prevent the straight use of EoS developed for the description of propertie above the glass transition temperature. Among the attempts to extend the use of EoS to glassy states, the NELF model [Doghieri and Sarti, Macromolecules, 1996, 29, 7885] resulted to be among the most successful. The model is based on the assumption the polymer mass density is a proper order parameter for the description of the out-of-equilibrium conditions of the polymer-solute mixture below Tg and that it behaves as an internal state variable for the system. In its early applications, the model allowed for the prediction of the pseudo equilibrium solute content in the mixture at assigned temperature and solute fugacity, based on properties of pure components, provided the actual polymer mass density for the system was known, which limited the predictive use of the model to the cases of light gases or low vapor fugacity. In a recent development of NELF model [Minelli and Doghieri, Ind. Eng. Chem. Res., 2012, 51, 16505] a simplified approach to the description of bulk rheology of the solute polymer mixture was proposed which allows for the prediction of volume swelling at pseudo equilibrium, based as well on pure component pVT properties of the polymeric species around Tg, and it proved to be effective in the description of volumetric properties of glassy polymer-solute mixtures in relatively large ranges for temperature and solute fugacity. In this work the use of model is specifically analyzed with reference to the prediction of glass transition conditions for vapor content and fugacity in the polymeric phase.