The stability limits of atom-vacancy solid solution are investigated in terms of the phase diagram for the alloy. It is shown that the interaction of vacancies accounted by the cluster model for the energy of vacancies formation leads to the asymmetric solubility region in the atomvacancy alloy. The simple scheme for the calculations of the phase equilibrium is suggested. The formation of complexes of vacancies is explained by formation of two-phase mixture in the atom-vacancy “alloy”. The mole fraction of atoms in such complexes is estimated.