Wow it's been a long time since I've been here. Guess I'll be useful.
The Vocaloid software is basically an instrument, and as such a user of a Vocaloid voice bank like Hatsune Miku can use it to synthesize a singing voice track for a song and then sell it without any worries about copyright. At the same time, the user gains full copyright of any works they create using Vocaloid, just like any other work they would have created, unless of course if they've waived those rights under a contract etc.
Where copyright comes in to play is when someone wants to use the image or name of the products, you would need permission from the owners. In the case of Hatsune Miku that is Crypton Future Media.
So if you want to sell an album and prominently display that you used a specific Vocaloid vocie bank, you'd most likely need to contact the company that made it and get permission.
For Miku however, and other Japanese made Vocaloids, there is the doujin culture where most publishers tend to overlook small-scale commercial usages, though it's a grey area.
For Crypton made Vocaloids specifically, they are very open about allowing derivative works, and see it as a way to expand the culture. For this purpose they created the
Piapro Character License, and more recently they adopted the
CC BY-NC license for the original artwork of the characters.
So in conclusion, all rights for songs created using Vocaloids belong to creator that created those works unless transfered under a contract, and anyone that wants to use them, Like SEGA for Hatsune MIku: Project DIVA, needs to contact the creators ,or the label if they're signed to one, just like any other song.
Any commercial usages of the Vocaloids themselves, by name or image, needs permission/license from the company that created it.
) still can get in the game, beside being a cover of the song by the band Loituma.
