We investigate theoretically the prospects of ferromagnetism being induced by cation vacancies in nonmagnetic oxides. A single Ca vacancy ${V}_{\mathrm{Ca}}^{0}$ has a magnetic moment due to its open-shell structure but the ferromagnetic interaction between two vacancies extends only to four neighbors or less. To achieve magnetic percolation on a fcc lattice with such an interaction range one needs a minimum of 4.9% vacancies, or a concentration $1.8\ifmmode\times\else\texttimes\fi{}{10}^{21}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$. Total-energy calculations for CaO show, however, that due to the high vacancy formation energy even under the most favorable growth conditions one can not obtain more than 0.003% or ${10}^{18}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ vacancies at equilibrium, showing that a nonequilibrium vacancy-enhancement factor of ${10}^{3}$ is needed to achieve magnetism in such systems.