We investigated computationally the α-, γ-, and β-isomeric structures, relative stabilities, and the electronic and basicity properties of magnetic [V^IV₁₄E₈O₅₀]^12- (hereafter referred to as {V₁₄E₈}) heteropolyoxovanadates (heteroPOVs) and their heavier chalcogenide-substituted [V^IV₁₄E₈O₄₂X₈]^12- ({V₁₄E₈X₈}) derivatives for E = Si^IV, Ge^IV, and Sn^IV and X = S, Se, and Te. We used density functional theory (DFT) with scalar relativistic corrections in combination with the conductor-like screening model of solvation. The main purpose of this investigation is to introduce the structure-property relations in heteroPOVs as well as to assist the synthesis and molecular deposition of these molecular vanadium-oxide spin clusters on surfaces. "Fully-reduced" polyoxoanions {V₁₄E₈} and {V₁₄E₈X₈} are virtually comprised of [V^IV₁₄O₃₈]^20- {V₁₄} skeletons of different symmetries, that is, D_2d for α-, D₂ for γ-, and D_4h for β-isomers, which are stabilized by the four {E₂O₃}²⁺ and four {E₂OX₂}²⁺ moieties, respectively. Our DFT calculations reveal stability trends α > γ > β for polyoxoanions {V₁₄E₈} and {V₁₄E₈X₈}, based on relative energies and HOMO-LUMO energy gaps. The α-isomeric polyoxoanions {V₁₄E₈} and {V₁₄E₈X₈} with the high negative net charges may easily pick up protons at the terminal E-O_t and E-X_t sites, respectively, which is evidenced by strongly negative enthalpies of monoprotonation. Energetically favorable sites on polyoxoanions α-{V₁₄E₈} and α-{V₁₄E₈X₈} for electrostatic pairing with countercations were also determined. Among β and γ isomers, the hitherto unknown γ-[V₁₄Sn₈O₅₀]^12- and γ-[V₁₄Sn₈O₄₂S₈]^12- seem to be the most viable targets for isolation. Furthermore, these Sn-substituted polyoxoanions are of high interest for electrochemical studies because of their capability to act as two-electron redox catalysts.