While I agree with how Roxas, Naminé and Xion became their own person. Vanitas and Ventus do still have those links to the chain of memories that the original Ventus in kh union cross had. I find that this further complicates their situation compared with Roxas, Naminé and Xion's situation, where they all started as a blank slate with basically little to non of the original links of the chain of memories of their original somebodies (excluding Xion in this case as she was siphoning Sora's memories due to being a replica of him).
They both only have unjointed
broken links though, which is no different to what both Xion and Roxas had as an underlying base, although one difference that is indeed there is that Roxas got the broken links of Sora's memories only later
after Naminé started her memory manipulation with Sora in CoM. That's why he falls into a coma for a while in Days and both Xemnas and Saix speak about that "now much hinges on Castle Oblivion".
Roxas (and Xion)
having those links was what prevented Naminé from fully restoring Sora's memories and what in the end made it necessary for both of them to join with Sora in the first place.
In BBS Eraqus explicitly states that Ventus can't remember anything and Nomura deliberately made a comparison between Ventus' and Roxas' "zombie-phases", but the broken links of the old memories being possibly present in both Ventus and Vanitas doesn't have any impact on Vanitas still being his own person just like Roxas. The memories Ventus made in the four years between Vanitas' creation and BBS itself are solely his while those made by Vanitas are his own as well.
These remaining old memory pieces/links though belong solely to Ventus and have nothing to do with the newly grown heart that is the being called "Vanitas" just like Roxas doesn't really connect with the memory flashes of Sora's memories he gets to witness in dreams/visions.
Ventus' heart was nursed back to health and "formed" by Terra, Aqua and Eraqus so he could more or less fully function again, meaning he doesn't
need Vanitas to function.
Vanitas on the other hand got his heart nurtured and "formed" by Xehanort so he doesn't need Ventus to function either.
Both were functionable on their own so they don't
need the other. Sora and Roxas actually were the same...until Naminé's memory messing and the siphoning ability of Xion came in.
Vanitas and Ven don't seem to have this, as Vanitas can still feel what Ven feels to a extant, meaning the two halves of their heart are still somewhat connected as well as in the scene where Ven remembers the memory of his time with Xehanort before he was split in two, it seems that the chain of memories of both the light and the dark half of his heart are still connected.
Nonetheless both Ven and Vanitas are described to be "amnesiac" and "empty" respectively so their situation is comparable. The comparison has even be explicitly made already.
Sora and Roxas are also still deeply connected, as is shown during both CoM and Days (where do you think Vexen got the memories to make the Twilight Town card in CoM from? He got them from Roxas, gaining access through his connection with Sora, that's what he means with the cryptical "other side of your heart"-remark).
So of course Ventus and Vanitas are still connected, just like Sora and Roxas are, but
not through an actual functioning and/or intact memory chain.
It's in all cases, Roxas, Xion and Vanitas (I'm excluding Naminé because no one knows if she even got
anything to begin with) loose old links of a severed memory chain belonging to someone else which they
can't consciously access or in any form feel belong to themselves.
It cannot be any different in terms of connection anyways because Vanitas only gained his current appearance through his connection with Ventus. Ventus is connected with Sora and Vanitas gained Sora's looks through his own connection with Ven.
On a side note I do also want to make a point if Vanitas is fine existing by himself as a separate being as (based on the novel) he can only feel negative emotions and destroying the unversed (which spawn from him consciously or not) causes him pain.
My own idea of why at the end of BBS when Ven and Vanitas fused and Vanitas took control instead of Ven regaining most of his original heart seemed to be because of 2 factors:
1) It was because of the piece of Xehanort's heart inside Vanitas.
2) Ventus was resisting and didn't want to fuse with Vanitas and so they both became two conscious's in one body.
I think its a combination of the two (mostly (1) though) that lead to the outcome in BBS instead of what happened with Roxas and Sora at the end of KH2.
I also want to comment on how Sora's newborn heart at the beginning of BBS mentioned that Ven would win back the piece of his heart (his dark half) that he lost back. I don't think I have heard anyone mention this detail when discussing this topic, it may be brought up in KH3.
I'm sorry if I get some points wrong, I haven't been keeping up with Kingdom Hearts news as much as I used to.
Vanitas being fine existing by himself is actually outright stated and shown in BBS itself, as
a) Xehanort was perfectly willing to ditch Ventus on Destiny Islands and let him fade away without any concern that it would affect Vanitas and
b) Vanitas himself showed no qualms trying to annihilate Ventus and trying to do the "X-blade-date" with Aqua as a substitute instead.
One doesn't need any secondary sources for this one.
Ventus himself makes it pretty clear on Destiny Islands that he wants nothing to do with Vanitas and the declarations to undo their fusion as well as the X-blade were also pretty straightforward.
Roxas eventually gave in and let the merge happen, Ventus didn't, that's the difference.
Winning back what you lost may indeed be a theme, although it would just like with Sora and Roxas refer to only the parts that actually
belong to Ventus, which were in Sora's case loose memory links and which may also be the same in Ventus' as the proof that a damaged heart can be nursed back to health
without explicitly needing the original part back has already been shown in the games themselves, so it may have a more metaphorical meaning rather than literal.