The vocative case and comma splices

I would consider the second sentence borderline unacceptable, the direct address masking the comma splice. Better would be

Don't touch that, John. It'll explode.

The first comma is necessary in direct address. Although it gets left out in emails and texts and chat quite often, in more formal writing you would always use it:

Hello, Mary.
How are you, Mark?
What's up, Doc?
Did you find your slippers, dear?

Finally,

Don't touch that John.

would seem to be cautioning people not to touch a certain individual named John.

The point is in these instances that it is dialogue, and thus the text is a reflection of the idiomatic qualities of the speakers rather than technically correct English. If it is dialogue, and there is a slight pause, it would seem natural to insert a comma.

So "Hello.. Mary.. how are you?" as opposed to "Hello Mary; how are you?"

So when we talk about written dialogue we generally discuss how it can most accurately capture the cadence, stumbles, and import of the tone of the speaker, rather than what the speaker specifically means.

I think the technical properties would only take precedence in the vocative case in the rare instances where the writer addresses the reader in the vocative case, in some sort of imagined writer role (i.e. not the author speaking in a personal manner as his or herself).

When it occurs in the written record of direct speech, a vocative is normally what Larry Trask calls a weak interruption. He advises:

Use a pair of bracketing commas to set off a weak interruption which could be removed from the sentence without destroying it.

On that basis, the commas are appropriate in ‘Hello, Mary, how are you?’ and ‘I can't believe, Howard, that you've put the duck in there.’ The remaining sentence is different in that it contains two finite clauses which need to be separated, or else joined by some device other than a comma. They could be joined by the conjunction or: ‘Don't touch that, John, or it'll explode!’ But if that isn’t what the speaker actually said, the speech needs to be presented as two separate sentences. So, as Robusto has suggested, ‘Don't touch that, John. It'll explode!"

On a separate point, English has no ‘vocative case’. Mary, John and Howard are performing a vocative function in the examples given, but they are not inflected forms.