A physical model is proposed to enable a unified thermodynamic treatment of the vacancy-bearing solid and the gas. It also solves the long-standing controversy in modeling the vacancies in solids within the CALPHAD approach. The model parameters are related to quantities that can be calculated by first-principles or measured experimentally. The vapor pressure of tungsten is calculated using the proposed model and methods, achieving satisfactory agreement with experimental data. It is pointed out that the site fraction is an order parameter in the present model that can be used to describe sublimation. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.