"You are Marcus."
Translation:Tu es Marcus.
Question vs statement has nothing to do with it. "Es" is the 2nd person singular and "est" is the 3rd person singular.
You are Marcus. Tu es Marcus.
Are you Marcus? Esne tu Marcus?
The -ne turns it into a yes/no question.
He is Marcus. Is est Marcus.
Who is Marcus? Quis est Marcus?
It would be wrong. You're not addressing him, you're stating his name.
Think of the vocative as an exclamation that exists just to the side of the sentence, not technically part of the sentence itself.
Marce, is est Stephanus.
Stephane, is est Marcus.
Marce, tu es Marcus.
Stephane, tu es Stephanus.