The BB vocal synth feature is not useable. The quality is very bad and the intelligibility is bad.
I recently started using another company's vocal synth called Dreamtonics Synth V and am very pleased with the results. I did have to purchase the program plus 2 english voices (Solaria and Kevin).
I was able to create the synth voice melody based on my exported midi melody track from BB. After entering the phonetic words in Synth V I was able to then export a new audio file for each vocal track plus the new harmony track. I could then import these audio tracks into BB and synch them to my preexisting song and remix the entire song.

If you could look into offering this as a purchased plugin option for BB I think it would gain a lot of interest and use for people like me who don't have access to a singer regularly.