The concentration of site Vacancies (v) is analysed as a function of the concentration of interstitial nonmetallic (X) atoms inside cubic-metal (Me) crystals. Its increasing dependence is established. The a concentration may exceed the concentration of thermally activated vacancies in the ＇pure＇ F.C.C.-Me at the same temperature and over a wide interval of X-concentration. Factors assisting the formation of such X-induced v are the following: (I) a strong repulsion of interstitial X-atoms and site Me-cations (2) a sufficient solubility of X-atoms (or clustering that leads to their local accumulation in interstices). On the contrary, an application of the pressure decreases the content of the impurity-induced a. An influence of such a on instability of alloys, that may lead to their polymorphic transformations, is considered. A monotonously increasing dependence is established for the a concentration as a function of H concentration in F.C.C.-Fe. The gamma*-phase of F.C.C.-Fe-H is expected to be enriched with vacancies at high H-doping levels. For instance, that is important as a precursor effect of spontaneous deformation (＇quasi-liquid state＇) near the F.C.C. to B.C.C.-Fe transformation in H atmosphere.