Now this is when Days references come along. Xion was absorbing Roxas, but he retained his form, meaning that he was in constant changes but weren't noticeable up until late. What I'm trying to say is; Vanitas was absorbing Ventus but it wasn't noticeable, up until late in the game when Aqua faces Vanitas.
What are you on about?
Xion was never in a process of "absorbing" Roxas, the only thing she was absorbing was Sora's memories through Roxas. Had things been different, Xion would have absorbed Roxas in the same way that Sora did. Both Sora and Xion were incomplete, and he could only complete one of them.
But that has little resemblance to Ven and Vanitas anyway.
It's Ven that has a heart (albeit fractured), body and soul. The only thing we know Vanitas to be made of is the dark side of Ventus' heart.
At first I scoffed at the idea that Vanitas is a heartless, but the more I thought about it, the more I think Samhain has a valid point. Look at what Vanitas is: The dark side of a heart. He was animated by darkness in the process of liberating (part) of a heart from a vessel (Ventus). Are not those the same conditions under which heartless are born?
While MX's heart didn't make a heartless when he released it (because it found a vessel in Terra), look at what happened to Sora when he used the keyblade to release his heart- boom, heartless. MX used the keyblade to extract part of Ventus' heart, so what's the difference? Technically speaking, Vanitas shouldn't even be as powerful as other Heartless since he was only made from part of a heart (and Ventus doesn't even have that much darkness in him anyway).
His body does at least bear resemblance to Pureblood Heartless. Though, if he were a heartless, it also raises a ton of questions. Like how he has a face, for example, and why it was never established that he
was a Heartless.
And then there's the fact that Vanitas has retained his sense of self. He's much the same as Ansem SoD in that regard.
Overall, I think it's a worthy point of contention. Vanitas was not portrayed as a Heartless, but the logic is there.