Vanitas's body seems to be akin to the heartless.
If you witness his birth cutscene, you see a heart floating, darkness coalesces around it and forms into his body, similar to a heartless.
A clear-cut way to say the difference is that a heartless(emblem) is a heart composed of both light and darkness, encapsulated by the darkness to the point where it's body has been cast off, and the darkness has manifested around that heart. Pureblood are a different discussion.
ASoD is like that, he is a heart, with a body forged from darkness.
Vanitas on the other hand is not a normal heart surrounded by darkness, his heart ITSELF IS nothing but darkness, he's unique really, hard to describe.
He might be the purest form of a heartless, Apprentice Xehanort tried extracting the darkness from a person's heart and their hearts collapsed, and later when he came he found heartless. MX was able to rigidly rip out all of the darkness form Ven's heart with his keyblade(to the extent where it wasn't just darkness, but half of his actual heart containing that darkness), so I guess MX was successful in the same process that Apprentice Xehanort failed in.