وَحَرْفُ النِّدَاءِ قَائِمٌ مَقَامَ «أَدْعُو» وَ«أَطْلُبُ».
Translation: The vocative particle takes the place of “I call” and “I seek”.
This explains the underlying object-like relation. Some vocatives are nevertheless indeclinable in form and occupy an accusative syntactic position.