Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
vŏcātīvus, a, um, adj. [voco], of or belonging to calling: casus, the vocative case, in gram., Gell. 14, 5, 1 sq. al.
As subst.: vŏcātīvus, i, m., the vocative case, Charis. p. 11 P.; Prisc. p. 671 ib. al.