I hope we get a Evelynn Vs. Vayne event in the future.

It is not complicated enough. Noxus vs Demacia were about ideology and people had a lot good and bad to say about both of these states. Diana and Leona are Sun vs Moon (I loved their pre-retcon conflict tbh).

Evelynn doesn't have an ideology. She is pure egoism and bloodlust, toying with people and being angry when they die too fast. Vayne too doesn't have an ideology. She hates Evelynn and once she finds her, there would be no banter, no debate, just a murderous intent.

If anything, Vayne vs Lucian would be a better option about any VS event because Lucian's code of honor and paladinism clashes a lot with Vayne's more pragmatic point of view.

: Every time you let innocents die in your vengeful hunt, you create a monster that will be stronger than you.
: I am not listening to a man who almost perished because he did too much crying and not enough shooting.

It also puts things into perspective and lets the player decide: do they support fighting evil benevolently or do they believe in a more ruthless yet more effective demon hunting?

Evelynn and Vayne don't work for the Versus theme. Versus is about rivals with a good side and bad side to each character's morality. With Evelynn and Vayne, Evelynn is the obvious villain who killed her parents for fun, whereas Vayne is the vengeful child hunting her down; she might be merciless, but she's doubtlessly the hero in that matchup. There's no gray areas for comparing those motivations.