بِحَرْفٍ نَائِبٍ مَنَابَ «أَدْعُو»
Through a particle taking the place of the verb “I call.”
In the meaning of yā Zaydu lies a call comparable to adʿū Zaydan (“I call Zayd”), but the two forms are not identically parsed. The particle fills the communicative role of the verb, while the vocative noun follows its own rules.
A vocative particle may be explicitly present or understood from context. The phrase “expressed or implied” prevents defining all vocatives as necessarily displaying their particle.