vocative case vs oblique case

vocative case and oblique case both are nouns.

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

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

vocative case: 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 case include oblique.

vocative case (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