vocative vs oblique case

vocative and oblique case both are nouns.

Word	Noun	Adjective	Verb	Adverb
vocative	Yes	Yes	No	No
oblique case	Yes	No	No	No

As nouns, oblique case is a hypernym of vocative; that is, oblique case is a word with a broader meaning than vocative:

vocative: the case (in some inflected languages) used when the referent of the noun is being addressed
oblique case: any grammatical case other than the nominative

Other hypernyms of vocative include oblique.

vocative (noun)	oblique case (noun)
the case (in some inflected languages) used when the referent of the noun is being addressed	any grammatical case other than the nominative

vocative (adjective)	oblique case (adjective)
relating to a case used in some languages