The lattice-gas model of Schouten, ten Seldam and Trappeniers is extended to mixtures of polymers. This model allows for three options on each lattice site: a molecule of type one, a molecule of type two or a hole. It describes a binary gas-liquid system in the molecular-field approximation. Following Flory, the chain length r is introduced as additional parameter in order to express the fact that the molecules occupy several lattice sites. In this paper we introduce two Flory chain length parameters (leading to the model of Tompa) in order to describe polymer mixtures in the presence of their vapor phase. The model can also be applied to a ternary mixture consisting of two polymers in a common solvent.