The only reason that Ven could survive when his heart was split was because he merged hearts with Sora. ( and Ven's "light side" was larger than his darkness.) So i am thinking that Vanitas must have merged hearts with MX, which could be why he is apparently inside of YMX in KH3D. Idk if this has already been proven or if its completely wrong, but i thought that i would post my theory on this, because i remember a while back when everyone was wondering where Vanitas' heart went when Ven destroyed him.
That is a pretty reasonable theory which I do think has been brought up before, but that isn't really a bad thing.
What you say is very plausible
and also further hinted by the fact that Vanitas has golden Xehanort-eyes, indicating a connection with him.
(We can't see Vanitas's ears so we can't confirm if he also has the pointed ears like Xigbar and Saix).
Still, one minor thing there: When Ven's heart was shattered the first time after MX created Vanitas newborn Sora's heart and Ven's didn't actually
merge, they touched and formed a link.
It was after the destruction of the X-blade that Ven's heart was hurt
worse and then went into Sora's to rest.
EDIT:
the only thing that i have to say is that ven's light side was larger because ven did not had darkness in his heart... if you're saying that Ventus whole beeing (Ven + Vanitas) light was larger then you're wrong because the x-blade was formed and they had to me in, let say, simetry the light and darkness...
That doesn't make sense. Ven had no "light side" after Vanitas was removed, he was ONLY Light.
The broken awakening platform in the BBS-Prologue shows it. The part broken off was merely a bit more than a quarter, proving Ven's light was always larger than his darkness.
Read the Xehanort Reports again, even MX comments on how full of light Ven was and that he was too kind for his own good.
Ven has as little, if not even less darkness than Sora has.
After he was created Vanitas had his own heart of darkness which could develop and grow, just like Ven's did under Eraqus's care.
Their hearts needed to be at equal power for the X-blade to be forged, but that balance level had to be archieved through training.
