Equations are derived relating vacancy characteristics to the solidus curves of the binary-alloy phase diagrams. These follow from the assumption that the ratio of the activation energy for vacancy formation to the melting point temperature (the temperature on solidus curve for the given composition) is approximately constant in pure metals of the same structure and some types of their binary alloys. The values of the vacancy-vacancy and vacancy-solute binding energies thus calculated are in good agreement with experimental data. All the calculations are performed in the pair-interaction quasichemical approximation. At the same time these methods are shown to account for the so called size-effect and valence-effect and to extend the calculations also for other phase-equilibrium problems.