Vocatives and Case Assignment

Vocative is problematic because it's not even clear, it should be considered a case since it plays a discursive rather than syntactic role (although you could make an outside case for the syntax by saying it marks it as not nominative or accusative).

In Indo-European languages with preserved vocative morphology, it is generally considered to be part of the morphological case paradigm (e.g. 5th case in Czech or 6th case in Latin). But you could just as easily call it a morpheme with a discourse function and exclude it from the paradigm (as some people have tried with the Genitive.)

Most Slavic languages have at least remnants of the vocative morphology. For instance, the Russian God! 'bozhe'. But mostly, they just use the nominative form. Czech has a fully preserved and functional vocative morphology that can be used across all classes of nouns (including inanimates). Related languages like Polish or Slovak have all but lost all the vocative morphemes in their modern varieties although there's still some awareness preserved of the archaic forms by native speakers. Vocative is also somewhat preserved in Bulgarian.

Modern Greek also has functional Vocative morphology which was inherited from Ancient Greek. Latin, of course, is famous for the vocative (which even got mentioned in the famous Monty Python scene).

Definiteness is also sometimes used for a vocative function - e.g. in Albanian where it expressed through a suffix.

Analytic vocative is found in Irish where it is expressed through a preposed particle.