If only for a moment.....
^^^^^ I go with this. As nice as it would be for anyone singing for me to learn the entire vocal beforehand, real life doesn't work that way when you've got an hour to do 2 songs.Know what you're going to do before you do it. Construct your vocal phrasing anticipating that you're going to be recording this one line at a time, if neccesary.
It's actually not at all difficult to do a vocal in sections or line by line and sound good. It just takes a little thought.