i agree. if real ciel's soul was in there, wouldn't he be more... child-like? i mean he was killed at a young age and through he would change a little, he defiantly would not know that much about politics and things. also, if real ciel's body was being controlled by somone else's soul, who is it?
i could just be overanalyzing tho.
Woah I love this theory!
If we go by this theory, I would think Vincent will know that he won't 'live' long in that body, and he uses this opportunity to provide Our!Ciel the 'training' and 'education' that he wasn't able to provide while he was still alive. I saw some parents deliberately putting their children in a challenging position as a means to let the child gain experience and grow to become strong human beings (not my type of education method for my kids :/ but yeah it exist).
Plus, it make sense to be Vincent, because Undertaker seemed most attached to Vincent than with Real!Ciel tbh.
Undertaker said that it was impossible for a bizarre doll to make another's soul their own. By that info, it's impossible for RCiel to have anyone's soul but his own. Which is impossible, since Sebastian ate it. So my theory is that he's soulless, and only running on blood transfusions and episodes. He clearly has (or had) a desire, which made it easy for Undertaker to construct a cinematic record based on that. Not to mention, a big part of his desire is OCiel, who's both a mirror image of himself physically and more active than ever. Seeing a physical manifestation of his desire (being a strong big brother alongside OCiel forever) must have made it easier for him to imagine he himself had a future, since the OCiel he once knew would never have been outside playing cricket at Weston. He's seeing himself in OCiel's place. He desired for what OCiel made a reality.
Besides, if RCiel had Vincent's soul, Undertaker wouldn't have been so torn up about Vincent's body burning to ash. A person's body and soul are their own; it's not possible to mix and match, even if they were blood relatives. It's the same thing with Claudia; if it were as easy as putting a soul into a host body, "she" would be up and active.
It might be since Vincent does give a bit of sinister vibe. He might have wanted to live longer or finish more of his unfinished business (since he was an earl as well)
I saw an interesting comment somewhere on here, and I've started developing a theory. What if it is someone else's soul controlling Real Ciel, since his soul was sacrificed to summon Sebastian? Most of the people who's bodies where reincarnated by the Undertaker had a zombie-like attitude. Real Ciel seems too... alive. What if the soul of a dead relative of Ciel's possessed him? I saw another comment mention how he looked a lot like Vincent in this chapter's panels. Perhaps Vincent possessed him? But if he did, why would he be plotting against Our Ciel?