Vanitas doesn't have a backstory, He was born from Ventus' darkness and the timeline in BBS is oddly unspecific except that it's ten years before KH1, Who knows how long Vanitas hanged with MX, but I'm sure it wasn't too long of a period, all he did between The time he was released from Ventus was released the Unversed, And trained with Master Xehanort in secret... there's not much there for development or details...