Acid has some libraries of RnB vocals. They have “oohs”, “aahs”, “yeah-yeahs”, etc. I forgot the names of the ones I’ve tried (Ilona maybe), but I didn’t find them very useful.
Now there’s Vocaloids which is a little bit different. It’s synthesized syllables, or something to that effect, that lets you create custom synthetic vocals that sound pretty realistic. Check it out.