When Vanitas was created it was in no way only a "minor" injury. Master Xehanort also cut
off an entire part from Ven's heart. That shard became then the core of Vanitas's heart.
The injury was in fact so severe that combined with Vanitas taking almost all of Ven's energy, him being weakened due to the heartless attack earlier he was on the brink of dissappearing.
That was also the sole reason why MX even dumped Ventus on Destiny Islands. Sora's newborn heart connecting with him thus saved him from certain doom.
The second time when Ven destroyed the x-blade the injuries inflicted to his heart were much worse than the first time. It was so bad he fell into a coma because his heart couldn't control the body any longer. Yen Sid could not even sense Ven's heart anymore despite him being a retired master so that also gives a hint on how bad it actually is.
What Sign said is correct, not to mention Ven's heart is asleep the whole time inside Sora in order to bear the pain. His plea to stay with Sora were his last words for a long time as far as we know.
Furthermore the chamber of Awakening is only reachable with the help of Aqua, and it's in another realm as well (the realm of in-between).
Ugh, now I'm gonna be off topic, but if he's awoken, would retrieving Aqua to save him still be necessary? She's the only one who know the way into the Room of Waking, but is the same vice versa? Or can Ven just wake up and be all "-stretch- K, lets get outta here" ?
That won't work because the chamber is
unreachable without Aqua.
Also Ven's heart can't get there on its own because it can't move.
Nonetheless, the way
how Ven might be saved could be shown in DDD, while the actual rescue will most likely happen either in the middle or near the end of KH III.
