What Vanitas actually did is contradicting himself in KH 3 when looking at all his statements in vanilla KH 3 and Re:Mind together. Not to mention he talks about "black masses" inside Ven which he doesn't know the origin of and can't confirm if they belong to Ven, to himself or a third being entirely in the character files.Except Vanitas has quite literally said that he's not really part of Ventus at all. He quite literally said that he was hidden inside Ventus for a really long time and all Xehanort did was just drag him out of Ventus. IIRC, the JP dialogue even had him flat out state he was a separate being from Ventus entirely. The point is, it's not really pointing towards a hybrid situation at all. More like Vanitas was just a foreign invader who would hide himself inside Ventus before Xehanort dragged him out. From what we've seen of UX, it just falls in line with what Ventus did with one of those darknesses at the end and forced it back into his heart and forging their connection. So yes you can maybe say Vanitas did take something from Ventus when Xehanort took him out(and that was a part of his heart) but Vanitas' situation doesn't sound like Roxas' situation whatsoever.
Just because he says something doesn't mean he's right nor that he knows all the facts.
This points towards Vanitas himself not being entirely sure and him taking/having a part of Ventus actually would be quite a big deal because the sum of all the parts (one of the six "lesser" darknesses + part of Ven's heart) would be bigger and thus different than the original.
"Vanitas" as the being presented in BBS and onwards didn't exist until Xehanort created him as it was stated he's a "newborn heart" and that heart was shaped for four years by Xehanort.
The developments in Union X confirm that the sources from which Vanitas was created were not solely from Ventus, but it does not confirm that he's unrelated or unconnected to Ventus completely.
The parallels to Roxas are there even if they are not 1-to-1 as Roxas is also more than the sum of the parts (Sora's and Ven's hearts) that originally created him.