Here's a sentence from the Catholic Mass:
Agnus Dei,
qui tollis peccata mundi,
miserere nobis.
Why not agne? Shouldn't agnus be in the vocative? Note tollis and miserere in the second person.
Similarly:
Sanctus, sanctus, sanctus Dominus Deus Sabaoth,
pleni sunt coeli et terra gloria tua.
Why not Sancte, sancte, sancte Domine Deus Sabaoth? Note tua.
Earlier, in the Gloria, it says:
Domine Deus, Rex coelestis,
Deus Pater omnipotens,
Domine Fili unigenite Jesu Christe,
Domine Deus, Agnus Dei,
Filius Patris.
So, there are definitely vocatives in the Mass—five in a row in the third line. But notice Agnus and Filius in apposition with Domine—in apposition with Fili, even!
Is it time to send out an errata sheet for all those missals?