I'm writing a backing vocal for an already-recorded song. Since there are other melodic parts she'll be singing over, I'm going to give the singer a score to work from.
In this case, the singer will be singing oohs and aahs in between the verses of the song. Providing pitch information is no problem, but how much should I write in terms of what vowel sounds to make?
Should I write in suggested oohs and aahs or leave it open for her to improvise? What methods of conveying this exist already?