The vocative macte is a party to a particular Latin idiom that you may encounter. Macte is the imperative of the Latin second declension adjective mactus (blessed, honored, cf. Greek μακάριος).
The idiom runs like this:
Macte (estō) (virtūte): success attend your honor!
Now, the ‘standard’ rendering offered by A&G (^^^) is a little too translationese, in my view. Something like, ‘be blessed in honor’ would more closely attend to the syntax of each word.
Further, realize that both estō and virtūte are optional, but at least one of the two must be present (macte estō virtūte, macte estō, macte virtūte).
With just macte! we have a different idiom all together (blessed! — something like the English ‘fantastic’ or ‘awesome’ or ‘that’s great.’)
Finally, we should echo A&G’s hesitation about the fact that the quantity of the final -e in macte is indiscernible given the extant verse poetry that contains this idiom, and therefore it might actually be mactē, an adverb. It is a matter of scholarly dispute.
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).