The influence of monomer structure on the vapor-liquid coexistence curves of lattice polymer melts is investigated through Monte Carlo calculations. The extended lattice model used in the computations has been introduced as the underlying model for the lattice cluster theory (LCT), a thermodynamic theory for polymer mixtures in the liquid state. The LCT model is essentially a bare-bones united atom model defined on a cubic lattice. The computational method employed is a variant of the density of states Monte Carlo algorithm. Chain architectures analogous to those of polyethylene, polypropylene, polyethylethylene, and polyisobutylene are considered. The computational results are compared with the coexistence curves predicted by the LCT. Qualitative and quantitative disagreements between the theoretical and the computed values are observed, casting doubts on the reliability of the LCT in the study of the phase behavior of melts and solutions of polymers with structured monomers. (C) 2003 American Institute of Physics.