There is no real vocative case in English, and hasn't been since it became English.

One can find mentions of the vocative case, as if it were real, but these typically describe it inconsistently to other descriptions, so we see it described as a variation of the "Nominative," "Subjective," or other true cases familiar to scholars of Latin or Greek (usually).

Example 1: The vocative case is one of the four main cases in modern English. The vocative case is used to indicate direct address (i.e., talking to somebody directly). Names in the vocative case are offset using commas. Nouns and pronouns in the vocative case are identical to those in subjective case. However, they should be offset using comma(s). This is why the vocative case is distinguished as a case.

Example 2: Modern English lacks a formal (morphological) vocative case. English commonly uses the nominative case for vocative expressions, but sets them off from the rest of the sentences with pauses as interjections (rendered in writing as commas).

We observe that there's no actual inflection involved (except in the limited case of pronouns, and most of those distinctions are null), but only indications of what actually happens in spoken English, that we pause slightly when identifying the people we're talking to, perhaps a sign of respect, or perhaps a simple acknowledgement of the special relationship that exists between speakers and listeners. To crib from the Seder, "Why is this person different from all other persons?"

In almost all cases, the current English usage is described by analogy with one or another language which does have cases. So we see tendentious arguments that usually start with some variation of, "When in Latin they say XXX, it is in the ablative case, therefore the English ablative case is... YYY."

The fact is that English is almost completely "isolating," like Chinese, and uses various particles, or even elaborate phrases, to describe situations in which Latin speakers would simply inflect one word or another to indicate true grammatical roles.

Since Classical scholars typically think that both Latin and Greek are better than English, and much better than Chinese, of which they're typically ignorant, they do love to use twisted Latin- and Greek-identified descriptions of what it is that we actually do, which perfectly explains why many English-speakers become confused when people tell them that they're using this "case" or that.

It's true that English does have a few quaint relics of our case-burdened history as we developed from Proto-Indo-European through several variations in three main streams (German, Norse, and slightly French), but these are either the plural (which is pretty darned simple, usually, except for "oxen" and a few other relics currently being abandoned by almost everyone with any sense of the tide of history, since "oxes" makes perfect sense to almost everyone, and no one says "shoon" any more) or some quirky inflection of a pronoun.

We note that our collective abandonment of case has gone far enough that the distinction between it's (the contraction of "it is") and its (the possessive) is one of the most common errors that people make in casual writing.

Vive l'anarchie!

Woman, without her… man… is wild.
Woman, without her, man is wild.
Woman without her man is wild.
Woman without. Her man is wild.
Woman…. Without her, man is wild.
Woman without her man is… wild.
Woman. Without her, man is. Wild!

Punctuation is only a rough indication of how people actually speak, and people are quite creative in their use of language, our first and most precious performing art.

Given a (theoretical) choice, by far the majority of human beings would choose blindness over being deaf, which is worth thinking about. The selection of one (theoretical) alternative or another is usually very rapid.

Oh, Canada, and O Canada is rather difficult for many people, and in fact the anthems of both the USA and Canada begin with the exact same sound. Coincidence? Perhaps not. Back in the days of Middle English (when spelling was often a matter of personal choice) O and Oh were used interchangeably, and some people argue that the vocative "O" actually originated in the interjection, Oh! We hear a possible precursor to the vocative O all the time, in fact:

Oh! Fred! Did I tell you about the comet that's going to destroy the Earth in seventeen minutes?

Okay, so we don't hear that particular sentence every day….

None-the-less, almost every interjection implies a listener (who may be absent or undefined) but especially "Oh!" because they are typically bids for attention.

Damn! Fred! Did I tell you about the comet that's going to destroy the Earth in seventeen minutes?

Is that the beginnings of a "vocative damn?"

I dare say that the "O ye of little faith…" quote is fairly familiar to many Christians, but that doesn't make it any the less archaic. We often use archaic forms in English to imply a certain solemnity:

Well done, thou good and faithful servant….

Perhaps that's a vocative "thou…."