The purpose of the study was to consider isothermal vapor-liquid diagrams of quasi-simple systems and to develop a universal algorithm for the calculation of isothermal vapor-liquid diagrams of these systems independent of the type of valence of the electrolyte, the number of components in the system, and the types of solid solutions. The suggested analogues of the three Gibbs–Konovalov and Gibbs–Roozeboom laws are true when moving along the univariant equilibrium lines on the solubility diagrams of systems with a random number of components. The study did not involve any experiments. The suggested algorithm was applied for the description of solubility (solid-liquid) diagrams and vapor-liquid equilibrium diagrams of three- and four-component systems with one, two, or three volatile components. In all the cases, the results of thermodynamic first-principles calculations agreed well with the experimental data presented in the literature. Both the experimental data presented in the literature and the results of the thermodynamic first-principles calculation performed by the authors are also in good agreement with the suggested analogues of the Gibbs–Konovalov and Gibbs–Roozeboom laws.