grammar - What's the subject in "You, go to the store"?

In the following sentence, what is the function of "You"?

You, go to the store.

I know the sentence is in the imperative mood, and that generally means there is an implicit second-person subject. If we dropped the initial "You", the subject would be that implicit subject.

While "you" matches that second-person subject, that seems just incidental and it seems like it is just an expression of address that happens to match the implicit subject.

Is it the subject, or merely a vocative expression? Or in other words, is the subject the "You" in the sentence, or is it the implicit subject from the imperative mood?