Looking through the assets on the Final Fantasy XIII game disk data, Japanese website MADOKA Park! apparently noticed something. Something naked.
After looking through the disk and turning up character models for all the main characters as well as main Summons and weapons, the site discovered a character model for Oerba Dia Vanille, and she does not appear to be wearing clothing.
MADOKA Park! confirmed with a friend who had cleared the game that no Vanille scenes that would use a model like this appeared in the game, leaving the site to wonder if this is a test model and to burst out laughing.
However, as it has been pointed out, there is a scene where Vanille transforms from a crystal back into a human. She is naked (but covered) during the transformation. This, it seems, would be the character model for said scene.
Everything makes sense!