In recent work a theoretical model was developed for the liquid-vapor phase coexistence of alkali fluids. The non-additive interactions due to valence electron delocalization are a crucial feature of the model. Originally the theory was implemented within a lattice-gas approximation. Now the results obtained, removing the lattice-gas description, with a Gibbs-ensemble Monte Carlo simulation are reported. The interaction energy is split into a repulsive part and an attractive part, which depends on the local environment of each ion. The results for cesium exhibit good agreement with experimental observations.