A Gibbs ensemble algorithm is implemented for mixtures of homopolymers and copolymers in a cubic lattice with coordination number z=26. We consider symmetric binary mixtures of homopolymer chains and also study the influence of the presence of symmetric diblock copolymers on these mixtures. All the chain lengths in a given mixture are identical (chains up to 128 units are included). A certain proportion of vacancies is introduced in each simulation. The phase separation curves are characterized by fitting the coexistence data to theoretical expressions. The dependence of the critical temperature on chain length, fraction of vacancies and also on the volume fraction of the copolymer in the mixture has been analyzed. Despite differences between models, the conclusions with this algorithm are similar to those obtained from previous simulations performed with techniques in which finite size interfacial effects may be important. Since the computational cost of our algorithm is reasonable, the present method can be considered an efficient way to study phase separation in different types of polymer mixtures.