Would Ven even accept Vanitas as a brother after all the bad things he did to himself and his friends? He tried to kill Aqua and based on the recent trailer will attempt to do so again.
That's quite a lot to forgive.
It really depends on how everything will develop throughout the whole of KH III's plot but Ven is certainly not
obligated in any way to accept Vanitas as
anything beyond an abusive enemy.
Going by the whole fuss made about the "brother" issue if I have got it right from several sources off the net the English subtitles in the trailer apparently didn't convey the actual meaning of the sentence very well in the first place as while "kyoudai" can indeed mean sibling or brother, the part coming before it "deki no warui" can be literally translated as "bastard", "ill-/poorly-made" or "misbegotten" which confirms my earlier interpretation when the trailer came out that Vanitas is literally doing the same he did during all of BBS which is insulting, demeaning and putting down Ven.
Thus, as of the
current state of information and stance of characterisation, Ven has no logical reason to accept or forgive Vanitas in any form as he just keeps doing the same shit he always did before.
A further thing is, the more heinous deeds and hurting actions pile up, the more difficult forgiveness and acceptance should become when things go to their logical conclusion as it is simply not human to be an all-forgiving doormat.
Vanitas was still a part of Ven. It isn’t just forgiving Vanitas as an individual but also about accepting a side of himself that always existed.
Except that the Vanitas we know is a product of Xehanort's grooming and not something that was always "a side" of Ventus himself.
He was "empty" at the start like Roxas was so there is no "Ven side".
Vanitas as a personality, the views and opinions he exhibits didn't exist before the whole heart-split thing and Xehanort himself does point out that Ventus was already a good kid before everything went down.
Why would Ventus need to accept something that was created and developed by Xehanort?
That a small part of himself was used as the base for it doesn't really mean that he is obligated to take everything it was turned into and connect it with himself.
If you have a tumor or other infectious tissue you also seek to cut that part of yourself away to preserve your own health and do not keep it around.
Hearts can evolve and develop independent of each other as both his and Vanitas' hearts did for more than four years which is more than Sora and Roxas ever had and with them there is much effort done to establish them as their complete separate people.
I have a lot of respect for Ienzo atm since he is like so far the only Organisation member who isn't part of the RAX trio to not be a SOD for KH3 and is actually helping Sora this time. But I still wonder what happened with Dilan. Aeleus may be good now still and hasn't got the SOD treatment?
Who says that he is actually genuinely helping Sora?
Xigbar and Ansem SoD speak about having "laid out a path" and Vexen too rambles about a "master plan" so everything Ienzo does might be done as part of a wider long term plan by Team Nort, no to mention that he seems to collaborate with Vexen and Demyx who obviously are Team Nort thanks to the eyes.
Granted, Ienzo may be not deliberately working towards Xehanort's goal but there are several people who are helping Xehanort without really wanting to.
Ansem SoD and Xemnas "looking forward" to Roxas' possible return is also suspicious.
---
As for the general topic on redemption I'd say I care more about that any redemption arc for a villain is executed believable with enough logic behind it and also actually goes the full way through, not just a "villain shows remorse for 5 seconds and gets immediately forgiven and accepted by everyone on team hero"-quick variant which induces only eye-rolling.
The meaning of Redemption is that one has to pay back a debt.
If you hurt people, you owe them.
If you wanna redeem yourself, you make amends.
That is what it is all about and a simple verbal "I'm sorry" doesn't cut it depending on how much damage was done.
It requires genuine efforts
on the villain's part without the heroes and/or victims having to pay any forgiveness or validation in advance and it is also not the villain who decides if their efforts are enough or not, but those they victimized.
That being said, I can theoretically see a possible redemption arc for almost everyone except Xehanort, and yes, that includes even Isa/Saix, Vanitas and Braig/Xigbar, although some of these I think would require more effort and a more convincing narrative than others for me to accept it as a genuine natural development.