The vocative sometimes signals a predicate NP in an address or an exclamation.
This construction is limited to poetry.
σύ τ’, ὦ ποτ’ οὖσα καλλίνικε, μυρίων
μῆτερ τροπαίων, Ἕκτορος φίλον σάκος,
στεφανοῦ
ὄλβιε κοῦρε γένοιο, τίοις δέ με τόσσον, ὅσον περ
Δᾶλον ἐτίμησεν κυανάμπυκα Φοῖβος Ἀπόλλων
Blessed art thou, young man, and may thou honor me as much as Phoibos honors Apollon Delos with his dark diadem. [provisional translation]