...I get that you like Namitas (Vanitas and Namine). However, your reasons for finding VaniXi as weird boggle my mind. And this is why.
Okay, that you think Vanitas would be too cruel and sadistic with Xion and not Namine raises HUGE alarms in your characterizations of Vanitas. I honestly can't believe I'm reiterating this, especially for someone who ships him with other canon characters.
VANITAS. IS. NOT. FLUFFY.
Perhaps in that creepy and disconcerting way you'd find some psycho right before he stabs you to death, maybe. But Namine wouldn't be exempt from his whole, "I'm a fucking psychotpath! 8D" side. Which is, you know, his whole thing.
If he wouldn't be fluffy with Xion, he won't be fluffy with Namine. In fact, he'd possibly be WORSE with Namine because she's so full of light and has a lot of weak points to hit.
TLDR; people destroy Vanitas' characterization to pair him with another character they like in a way that goes completely against everything that makes him Vanitas.