That may have partially been my fault. Sorry. I tried to downplay the VA debate by excusing it for asking for better writing around page 2 of this thread.
I'm actually of the opinion that they're indirectly related though. Better writing might result in better deliverance and weight of lines. However, I also agree with what Audo said; sharing VAs between two characters usually results in one of the characters falling short of the mark (no, this doesn't mean I support the idea of Kairi and Xion being voiced by two different people; it goes against canon). It would've been easier for the VAs if the connection between the two shared characters were trivial, as there have been cases in other media where two characters shared VAs and pulled off the deliverance of each character well due to not having connections (I'm looking at South Park here, but there's a lot more), but as it is, that's not the case; the connections run deep, for Sora and Vanitas, Riku and his replica, Kairi and Xion, Ven and Roxas, etc.