What is the difference between a vocative and an epithet?

My dictionary defines epithet as follows:

n. an adjective or phrase expressing a quality or attribute regarded as characteristic of the person or thing mentioned: old men are often unfairly awarded the epithet 'dirty'.

So I don't see how either of your examples qualifies as an epithet.

Vocative denotes a case of nouns, pronouns, and adjectives in Latin and other languages, used in addressing or invoking a person or thing.

Perhaps the most famous vocative of all time were the words Shakespeare attributed to Caesar as he was stabbed by the conspirators Et tu Brute (Even thou, O Brutus) - Brute being the vocative of Brutus.

Hence I do not think either of your examples qualify either as epithets or vocatives.