So it's pretty much like this: Each Vocaloid has a voice bank. In that voice bank, is every Japanese syllable at each pitch that the Vocaloid can sing. From there, I think you pick and choose the syllables and pitches you want them and in the end, you end up with a song. Well, the singing part of the song. As far as the accompaniment, I'm not sure.
@Neko-kun, I see. Only about three more days till school starts for me! ^^ Can't wait!