Halliday & Matthiessen (2014: 159):
Another element that figures in the structure of the clause as exchange, but outside the scope of the Mood and Residue, is the Vocative. This also is fairly mobile, occurring (a) thematically; (b) at the boundary between Theme and Rheme (not usually between Mood and Residue) or (c) clause–finally; and with the same intonation patterns as the comment Adjuncts. The Vocative can accompany a clause of any mood, but it is relatively more frequent in ‘demanding’ clauses (interrogatives and imperatives) than in ‘giving’ ones (declaratives).