I don't believe Vanille is a traitor. If so, I commend Square Enix for such an interesting twist to the story if she does turn out to be one. She just seems like a victim of herself.
She must have done something that is significant enough to offer grief to Serah, and even Sazh. It may be something to do with the crystallised Lake Bilge and the Pulse fal'Cie (Vanille could even be originally a Pulse habitant).
Let's not forget based on the novelisation that it may have been the Pulse fal'Cie that transformed Serah into a L'Cie. It may also be that same fal'Cie in the crystallised lake. Whether this affects Serah in any way is unknown, but Vanille seems remorseful about something.
She must have done something that is significant enough to offer grief to Serah, and even Sazh. It may be something to do with the crystallised Lake Bilge and the Pulse fal'Cie (Vanille could even be originally a Pulse habitant).
Let's not forget based on the novelisation that it may have been the Pulse fal'Cie that transformed Serah into a L'Cie. It may also be that same fal'Cie in the crystallised lake. Whether this affects Serah in any way is unknown, but Vanille seems remorseful about something.