My opinion on the matter is that these things are generally in a state of flux. Everyone has the capacity to grow, no matter the direction. The same way villains in faerie tales have their dramatic descent into madness, Vanitas might have his own slow ascent to something a bit more well-rounded.

Are you not brothers, though, if not by blood? Isn't connecting your hearts kind of like that on a spiritual level?

[Ah, the lovely worldbuilding dynamics of Square-Enix games...]