Birth by Sleep Dark Memories - Vanitas's origins according to the novel.

Link

Not sure if it's been posted, saw it on GFaqs and thought I'd post here for people who didn't.

Personally, i loved it, made Vanitas character way better, and it kind of explains a lot of things, such as why he's doing stuff with Xehanort with no apparent backstabbing in the works.
Though it's probably not Canon(Maybe not just yet), since in one of the novels Roxas used O&O in a time were he clearly shouldn't be able to.

... Also disregard the fact that it's a fanfiction site.

 

The first thing I saw in your post was "Also disregard the fact that it's a fanfiction site." So I clicked the link and went to FF.net. I was all liek "Oh, well, maybe later."

>_>