On the S910 on up, can't you record vocals this way?

1. Record your keyboard performance, and save as a .mid file.

2. Make any necessary adjustment that is needed for audio recording.

3.Play back that .mid song, and while you sing along, record and save it all as a .wav

This of course means you need to pre-balance sounds in the saved .mid as to not drown out the future recording of the singer. (Different than if you were recording and saving an Instrumental). Usually at a point in the song, the singer takes a break, and lets the 'band ' take over. At that point you need to up the volume of the instruments, until it's time for the vocalist to come back in. Some planning ahead will make for a decent recording, all on the keyboard. Of course it won't hurt to fine tune the .wav on a computer program, before saving as an .MP3

One pet peeve of mine (and probably many) is when the singer has this need to sing every note in a song, without letting the band have a solo turn. Somewhere, singers should always take a break in a performance. it makes the song and the singer much more professional like.