Well, let’s have a look into a Grammar:
These are really adjectives of the First and Second Declensions, and are so declined (see §§ 110-112). But meus has regularly mī (rarely meus ) in the vocative singular masculine.
(Allen and Greenough’s New Latin Grammar for Schools and Colleges, §145)
“rarely meus” and this is only used in special circumstances such as:
With older authors:
sed, amabo, oculus meus, quin lectis nos actutum commendamus? (Plautus, Persa 765)
Da, meus ocellus, mea rosa, mi anime, mea voluptas,
Leonida, argentum mihi, ne nos diiunge amantis. (Plautus, Asinaria 664)
Also sometimes with Augustan poets:
proice tela manu, sanguis meus! (Vergilius, Aneis VI, 835)
Until that time the examples have in common that meus is only (and rarely) used with nouns that use the nominative form for the vocative (either because there is no special form for the vocative as with sanguis or because the nominative form is used, which is another rare usage. The examples for this are oculus and ocellus).
In later times there is a little more variation but meus is still rare with a vocative. Another important point is that all these examples are taken from poetical texts. Meus for mi is as far as I know not found in the texts of Cicero or Caesar for example.
So to summarize: You should rember as a rule that mi is voc. sg. m of meus but bear in mind that there exist some special exceptions that should not be imitated if you want to produce standard Latin prose.