We report a study of metal (M = Fe, Co, Ni, and Ga) substitution at Cu sites in small quantities (x = 0.0, 0.03, 0.045 and 0.06) in fully oxygenated (y = 0.09) and oxygen deficient (y = 0.24 and 0.32) polycrystalline ErBa₂Cu_3-xM_xO_7-y. In contrast to Fe, Co and Ga, the site preference (Cu-O chains/CuO₂ planes) for Ni is found to depend on the oxygen content of the host system. The substitution of Fe, Co and Ga (and Ni in an oxygen deficient set of samples), with respect to the pure (x = 0) samples, results in a 10-45% reduction in orthorhombic distortion and a negligible (y = 0.01-0.03) change in oxygen content. The analysis of these variations suggests the formation of oxygen vacancy disorder in CuO₂ planes and/or in Cu-O chains, which may have an important role in determining the striking changes observed in a host of properties of ErBa₂Cu_3-xM_xO_7-y.