I was taught that meus had a special irregular vocative, mī. (So "my father" in the vocative would be pater mī, not pater meus.)
However, there's a line that shows up a few times in the Vulgate that seems to contradict this.
Deus, Deus meus, respice in me: quare me dereliquisti?
God, my God, look back to me: why have you forsaken me? (Psalms 21:2 or 22:2 depending on version)Et circa horam nonam clamavit Jesus voce magna, dicens: Eli, Eli, lamma sabacthani? hoc est: Deus meus, Deus meus, ut quid dereliquisti me?
And around the ninth hour Jesus cried out in a loud voice, saying, Eli, Eli, lamma sabacthani? That is: my God, my God, why have you forsaken me? (Matthew 27:46)Et hora nona exclamavit Jesus voce magna, dicens: Eloi, eloi, lamma sabacthani? quod est interpretatum: Deus meus, Deus meus, ut quid dereliquisti me?
And at the ninth hour Jesus exclaimed in a loud voice, saying, Eloi, eloi, lamma sabacthani? Which is translated: my God, my God, why have you forsaken me? (Mark 15:34)
The conjugation of the verb makes it clear that the speaker is addressing God directly. So why do all these instances use meus instead of mī?