In this paper we report density functional theory (DFT) calculations on bulk cerium oxide (ceria) doped with magnetic impurities of cobalt atoms in the presence of oxygen vacancies. Using the framework of the DFT + U approach to take into account the effects of electronic correlations in the Ce 4f states, we evaluate the relative stability of different configurations of vacancies. We show that, within the approximations considered, the vacancies tend to locate close to the Co impurities. In addition, we address the issue of the charge localization that takes place due to de-oxygenation processes, finding that the excess electrons reside at Ce atoms which are next-nearest neighbors of the vacancy sites.