In that case, it really depends how much freedom you want to give them to add their own interpretation. If you want them to follow something down to the letter, including every little phrasing and flourish, you will likely have to given them a pretty polished vocal demo. If you want them to completely do their own thing, you can give them nothing at all and tell them to just go for it. And there is lots of room in between those two ends of the spectrum, such as a rough vocal demo, you playing the basic vocal melody on an instrument, etc.
I once auditioned for a band years ago where they gave me an instrumental track and proposed name of the song, and told me to write lyrics and my own vocal melody, and be prepared to sing it at an audition a week later. And it was complex prog like DT, with intended instrumental sections, etc. So it was really a challenge to figure out all on my own what to sing, how to sing it, and even WHERE to sing (and not sing). As the auditionee, I did not feel like that was the most productive approach. But that is what they wanted--someone who could come in and write melodies and lyrics from scratch to their satisfaction over their music.