I still very much doubt that and besides some coincidental similarities there is no real evidence.
The Twin Theory was obvious, but what i read about this "Theory" so far just hangs in the Air.
Or to be more specific, its more than that. While knowing how Yana Toboso writes, she has more in stock by just simply saying Vincent is Sebastion, lol.
It could also be Sebastian serving Vincent as a Demon as well or the whole Situation about Claudia Phantomhive and why Undertaker (kind of) serves the Phantomhives.
Could it be Vincent had a contract with Sebastian before? And the seal was on his hands that is why he always wore gloves?
Maybe, maybe not. Wearing gloves regularly was normal at that Time. It shows nobility and etiquette. Our Boy is wearing them quite often as well.
Uh... that's obviously emergency exits that Ciel and Sebastian made.
If only Sebastian knows about it, that would certainly shock Ciel.
Also.. this manga was planned as a yaoi originally, so I seriously doubt Toboso wanted an incenst story.
But she even pull out an evil twin plot from nowhere so.. what would I know?
Once again, Sebastian shows he is Vincent. Heremembers things only Vincent knows about. I mean photo studio and secret word. Toboso killing me with hints