The novels are treated as secondary-tier canon often, meaning they take a backseat to interviews and game-canon (which is obviously the highest).
I won't say it is impossible for an element of the novels to be expanded upon and become actual canon by referencing it in an interview or even a game itself. After all, Nomura did give some input on the novels.
With that Vanitas-bit I am sceptical however because of the form of that beings eyes, they're identical to a dream eater nightmare.
Furthermore, Vanitas reverting to his appearance before Sora's connection with Ven's heart makes no real sense in several ways. While Ven's heart may be extracted from within Sora's, the connection between their hearts will remain since they are so closely linked together ever since Sora was born.
On top of this Vanitas, like Roxas, remembers how he looked like while he was "alive" in the physical world, so his appearance in the RoS would correspond to how he remembers himself, just like already shown with Roxas and Naminé.
A heartless is formed when a heart is overtaken by its own darkness (pureblood) or captured by darkness (emblem). Vanitas's heart is darkness itself, pretty much impossible to get a heartless from that.
EDIT:
Congratz on going silver, Kupo1121.
