According to Spideraxe's twitter, Vayne was updated on PBE to have a 1.5% AP ratio on her W.
Thematically speaking, Vayne wouldn't tolerate people who use magic, or using magic to kill monsters. As a character, she's vehemently anti-magic, so much so that she would kill her mentor and mother figure upon seeing magic being used to save Vayne's own life.
Vayne put an arrow through Frey’s heart without allowing her another syllable. Whatever affection she had felt for Frey evaporated upon discovering her true nature. A tear formed in Frey’s eye as she collapsed, but Vayne didn’t notice – whatever warmth the two had shared died with Frey.
...
For the first time since her parents’ death, Vayne smiled.
Even though a lot of marksmen items in the PBE patch 13.10 have hybrid scalings, her kit is sufficient enough such that I don't think Vayne should receive AP scaling.
That being said I'm not a Vayne player. I just like small quirks in the game that reference a champions lore.
Edit: Much like Vayne, I’ve experienced some character growth and have learned 3 things:
1: I see now that Vayne uses sentinel weapons and she doesn’t despise non dark magic.
2: AP doesn’t necessarily mean magic/ magic damage.
3: I saw one comment that said something like “I dont care about lore, I just wanna see some rabadon vayne on the rift” and now I’m in favor of not only AP Vayne but also random scalings on every champion.
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