What does a vocal arranger do?

A vocal arranger decides how the artist will sing a song. It might be a line that the arranger thinks sounds better if the singer uses a vibrato in their voice, or a wavery tone rather than straight. Maybe the note should be sung an octave higher, or add a breathy tone to the words. He may say "Hey, add a yell, right there, or an ooohh aahhh."