A&G define the vocative as “the case of Direct Address.” (35f)
Generally speaking, the vocative and the nominative are the same.
However, in certain nouns of the second declension (those with nominative -us or -ius) have two exceptional variations. All nouns in -us feature an -e in the vocative (mūrus…mūre). Those ending in -ius (Vergilius, fīlius, genius, etc.) take a vocative -ī (Vergilī, filī, genī).
- [Highly attentive readers should note that this vocative does not shift its accent, rendering Vergílī, and not *Vérgilī, as one might expect.]
That’s how it stands for nouns. There’s a slight variation in policy for adjectives, though luckily the same general rule (same as the nominative) holds true for all but the second declension (bonus…bone). However, the one catch is that adjectives ending in -ius change to -ie and not -ī. Therefore, when calling to a Spartan son, we might say O fīlī Lacedaemonie! (not *Lacedaemonī).
If anyone has a better understanding of vocative plurals, which I assume are all identical to their nominative forms, feel free to say more in the comments below. A&G are totally silent on this issue, which I assume signals that listing the vocatives would be redundant (with respect to the nominatives).
The Essential AG: 38a
Latin for Addicts 
Ecce Romani (the book I teach from) teaches all vocative plurals as the same as the nominative plural (-ae, -i, -es, etc.), but I can’t remember what Jenny’s used to use.
Also, not all 2nd declension nouns take an -e as an ending. The vocative of “vir” is “vir”, as is the case with all 2nd declension nouns that end in “r” instead of “us”
Thanks! Also, thanks for spotting the distinction with vir/puer/ager/etc. My language above seems to suggest that all second declension words end in us or ius.
In Plautus in Comics, they use ‘puere’ for the vocative of ‘puer’ but I’ve never seen it anywhere that this is a rule or even a known variant.
Augustine in the Confessions, which is framed as a second-person address to God, consistently addresses him as “Deus meus.” I don’t know whether anyone does so before him.