<<Correction: not specifically in ancient times, but in several Indo-European languages, with the possible exception of Germanic ones, which seem to have been the first to lose the vocative case. >>
True: The Germanic vocative and nominative cases fell together early on; however, the adjective continued to demonstrate a type of vocative by employing the weak declension as in:
'god mann' - good man (regular nominative);
'goda mann' -[hey,] good man!/[thou], good man! (vocative)
'gode menn' - good men (regular nominative);
'godan menn' -[hey,] good men!/[ye], good men! (vocative)
True: The Germanic vocative and nominative cases fell together early on; however, the adjective continued to demonstrate a type of vocative by employing the weak declension as in:
'god mann' - good man (regular nominative);
'goda mann' -[hey,] good man!/[thou], good man! (vocative)
'gode menn' - good men (regular nominative);
'godan menn' -[hey,] good men!/[ye], good men! (vocative)