Vayne is getting a legendary skin with Ruination event and it revealed more than we were probably meant to know for the time being.
Riot Games update League of Legends PBE with Sentinel and Ruined skin lines and Vayne's skin is of Legendary tier, meaning she is getting new voice lines. SkinSpotlights picked up on the data and released a video with all of her new voice lines, some of which referenced two skins that weren't added with the initial batch.
The first one is Ruined Miss Fortune, which is bound to be popular, given the fan base's love for the character.
Meanwhile, the unrevealed Sentinel skin is for Pyke, which is a bit odd, given his serial killer tendencies and the fact that the voice line where Vayne references him also notes she doesn't trust the assassin support.
Considering that both of them were in the party that went to the Black Isles, it's possible this is where the skin line is hailing from. It's also possible that the game's story will explain how Pyke got invited into the Sentinels of Light and how Miss Fortune fell to the power of the mopey king's mist.