A method combining two approaches is proposed. The first one is based on the density-functional theory solution within the stabilized jelly model for a metal monovacancy ignoring an external surface. The second one uses a solution for a defect-free metal in the presence of an external flat surface, but with lowered atomic density. This lowered density of atoms appears due to the existence of superlattice of vacancies with a relative concentration $c_{\textrm{v}}$ within the defect metal. Using $c_{\textrm{v}}$ as a small parameter, all metal characteristics are expanded into $c_{\textrm{v}}$-series. The zero terms of the expansion correspond to defect-free metal, and linear-$c_{\textrm{v}}$ corrections are expressed through its characteristics. The consecutive procedure for the calculation of a size dependence of ionization potential and electron affinity for a large spherical metal cluster of radius $R_{N\textrm{,v}}$ containing $N$ atoms and $N_{\textrm{v}}$ vacancies is presented. Within the scope of the effective-medium approach, the perturbation theory over the small parameters $R_{\textrm{v}}/R_{N\textrm{,v}}$ and $L_{\textrm{v}}/R_{\textrm{v}}$ is proposed for electrons’ ground-state energy ($R_{\textrm{v}}$ is the average distance between the vacancies and $L_{\textrm{v}}$ is the electron–vacancy scattering length). The profile of effective potential, phase shifts of the scattered wave functions, and electron scattering length previously have been calculated by means of the Kohn–Sham method for a macroscopic metal sample within the stabilized jelly model. Obtained analytical dependences can be useful for both the analysis of results of photoionization experiments and the determination of the size dependences of the vacancy concentration including the vicinity of the melting point.