The formation of defect hydrides containing a large number of M-atom vacancies (superabundant vacancies; SAVs) was studied in bcc NbHx and in the fcc phase of FeHx, CoHx, NiHx and PdHx, by resistivity and XRD measurements under different conditions of hydrogen pressure and temperature, with/without allowing for exchange of hydrogen with environment (open-/closed-system methods). Two distinctly different behaviors were observed: in metals with small formation energy of Vac-H clusters, both H and vacancies enter abundantly into the M-lattice to form the ultimate defect-ordered structure, whereas in metals with relatively large formation energies, vacancy concentrations remain relatively small. This general trend was examined by Monte Carlo simulations based on a lattice-gas model. The result showed the occurrence of two distinct phases in the vacancy distribution caused by the combined action of the long- range elastic interaction and local Vac-H interactions, in accordance with the observation. Conditions for the occurrence of these 'vacancy-rich' and 'vacancy-poor' states are examined.