We simulated benzene adsorption in Na-X and Na-Y zeolites on a lattice of binding sites using a two-replica cluster Monte Carlo algorithm. Evidence for a vapor-liquid phase transition is explored for a range of guest-guest and host-guest energy and entropy parameters. The critical temperature is found to vanish precipitously with increasing energy difference between sites. For Na-X, critical temperatures as high as 300-400 K are found for reasonable values of the parameters, while for Na-Y no phase transition is predicted.