We consider a rarefied gas mixture confined between two parallel walls consisting of vapor passing through the walls (evaporation, condensation), and a noncondensable which is totally reflected at the walls. Under a diffusive scaling we derive a macroscopic limit in which the noncondensable forms a well-defined boundary layer slowing down the vapor flow. The results differ substantially from others obtained with asymptotic analysis strategies. Our calculations are based on discrete velocity models.