We present a lattice model for polymer solutions, explicitly incorporating interactions with solvent molecules and the contribution of vacancies. By exploiting the well-known analogy between polymer systems and the O(n)-vector spin model in the limit n0, we derive an exact field-theoretic expression for the partition function of the system. The latter is then evaluated at the saddle-point, providing a mean-field estimate of the free energy. The resulting expression, which conforms to the Flory-Huggins type, is then used to analyze the system’s stability with respect to phase separation, complemented by a numerical approach based on convex hull evaluation. We demonstrate that this simple lattice model can effectively explain the behavior of polymer systems in explicit solvent, which has been predominantly investigated through numerical simulations. This includes both single-chain systems and polymer solutions. Our findings emphasize the fundamental role of vacancies whose presence, rendering the system a ternary mixture, is a crucial factor in the observed phase behavior.