Vayne is absolutely fucked up in the head.

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I don't think it's as psychopathic as it seems, meaning I think she has a point. Evil, even when used with good intentions (Frey), or forced upon someone (husband guy), is still evil. Maybe she shouldn't enjoy killing them, but she might not be wrong to deal with black magics in absolutes.

I also think that the moral debate caused by the rigidness, but potential justification of her values, gives her quite a bit of multi-dimensionality.

I think the problem is that she became obsessed in her endeavours.

Everything she does is for revenge. Revenge engulfs, consumes, and takes over your entire life and being. It can get so bad that you become the monster you so desperately seek to destroy.

I'ma copy/pasta what I mentioned once or twice before.

If you ever seen Afro Samurai, you'll know that Afro is on a road to revenge. His father wore a headband that read #1. The world's second greatest fighter had #2. #2 killed Afro's father while the kid was only 5. The man then gave Afro the #2 headband and told him to "Let your hate flow through you." During his journey, he killed the man who had a son that was also 5. Afro gave the child the #2 headband and told the kid the exact same thing.

The story went full circle. Vayne is still smart. She is still calculating. She calculates all actions in the heat of the fight. She goes for flashy because she wants to see them suffer as much as she did. She's in love with it, engrossed by it, enamored of it. To the point that--to those she slays--she is like a demon, looking down upon her kill. Her lips now forming a lustful grin.

So yeah. I think her multi-dimension would be whether or not she'll eventually realize or question what she's doing.

We can also bring up Lucy and MF's interactions in Blood and Fortune. Remember, MF was so obsessed with her revenge with GP, she pulled all of Bilgewater into chaos. To make matters worse, after she killed GP, she went on a rampage and sought out the deaths of every single man who ever worked for him. She was a tyrant. She was a monster. She was a reaver not too unlike the previous king himself. Afterwards, she felt empty inside. All that she done did not bring her the satisfaction she thought she would have, and all she knew was going to be swept away by the Harrowing. That's why MF and Lucy has a special connection. He was headed down the same path she was taking. He lost a love one too, and he wanted that fart cloud's head on a platter.

Unfortunately, Vayne's a bit too sadistic and fetishist over her desire to purge all dark magicks.

Yes, I believe that's what one refers to when they say "seeing things in black and white" or that she's one-dimensional. She literally displays all of the characteristics that real-life psychopaths display, but even if she somehow wasn't a psychopath, one-dimensional black and white characters are boring. Also: http://tvtropes.org/pmwiki/pmwiki.php/Main/DarkIsNotEvil

Her own "justification" for her actions is that she enjoys it. That seems to be stated pretty explicitly in her lore. There's being "rigid", and then there's having a literal one-track mind. When Vayne killed Frey, there should've at least been some sort of inner conflict, but instead she somehow magically overrides her feelings and just behaves ridiculously in an instant. That is not a believable or interesting character.

I dislike Vayne for the same reason I dislike Garen. As far as I could tell in his lore, he no thinkie with his brainie, then Lux pretty much had to fix everything, and even after that he still keeps his black and white mentality. Of course Vayne is a tad bit more extreme in that she completely disregards her own emotions somehow, and Garen is just an idiot.