In a recent publication [J. Chem. Phys. 113, 5401 (2000)], we investigated the applicability of ab initio pair potentials for acetonitrile and methanol for predicting their phase behavior using Gibbs ensemble Monte Carlo simulations. Here, we extend this study by introducing improvements to the pair interactions to better represent the phase behavior of acetonitrile and methanol. The first adjustment was a scaling of the interaction energies so that the calculated second virial coefficient matches the measured values. Excellent agreement was obtained for the second virial coefficient by scaling of the pair potentials by 1.08 and 1.20 for acetonitrile and methanol, respectively. The predicted phase behavior with these scaled potentials results in a large improvement for methanol, and slightly poorer predictions for acetonitrile. Next, to account for multibody effects, a classical polarizable model was added to the simulations, which showed that multibody interactions do not contribute to the equilibrium properties of acetonitrile, but play an important role in the molecular interactions of methanol. The addition of polarization terms to the simulations for methanol results in vapor pressures in good agreement with experimental values.