Okay so this is me theorizing on all the KHUX Darkness lore now we know or can heavily assume Vanitas is one - the one they fought in data daybreak town, the one who killed strelitzia, etc.
so we know Darknesses lose their will, their sense of self and memories, the longer they're floating around outside a host body's...well, body and Heart.
we also know Ven used his light to "reshape him into a form they could defeat", which i am gonna assume meant literally giving him a physical form = the red and black dark suited form he emerges as when xehanort rips him out of Ven's Heart in BBS. he was already like that BEFORE Ven connected to Sora, so that tracks.
but my real meaty question here is this: is this body Ven gave him just some "psuedo body", or does it actually count as a "physical form" that allows Vanitas to exist outside of Ven/his host's body without degradation of mind and self?
we don't know how long it would take a Darkness to start losing themselves - it could take like ten years, or maybe a hundred.
but we do at least know Vanitas existed fine for the FOUR YEARS he was apprenticed to Xehanort, when Ven was sent to Eraqus.
so that is maybe proof that the "body" Ven created for him DOES count as a valid physical form that keeps a Darkness's will from fading.
OR four years isn't enough for the process to start, and Vanitas was actually done a favor when Ven beat him in the end of BBS and took him back into his Heart in his pure darkness form.