The concentration of vacancies (v) is analyzed as a function of the concentration of interstitial impurity (X) inside a close-packed metal (Me). Over a wide interval of X concentration, the v content exceeds the concentration of thermally activated vacancies in the 'pure' Me at the same temperature. For instance, a monotonously increasing dependence is established for both v concentration and f.c.c.-Fe-cations' self-diffusion coefficient as functions of H concentration. The gamma(*)-phase of f.c.c.-Fe-H is expected to be enriched with vacancies at high H-doping levels. That is important for fast Fe-diffusivity as a precursor effect of spontaneous deformation ('quasi-liquid state') near the f.c.c.-to-b.c.c.-Fe transformation in H atmosphere.