nobody wants to make the game fun, they want to gain ELO... people just realize that this champion is so insanely safe in the early game because you can camp for her and just completely bypass her weak early game
people just want to tilt the other team, and that's exactly what vayne does... she tilts people with her presence
in my opinion, she is the MOST toxic champion in the game because at the very moment this champ is picked, the ENTIRE game becomes about her. How do you shut down vayne? Are we being aggressive enough against vayne? Are we camping Vayne? Are we camping for Vayne? Let's group on Vayne's power spike. Give vayne red even tho she's 0/2 and our Xin is 5/1. Vayne needs it. We need to pick lulu and janna to support vayne, she'll carry us. Peel for Vayne, dive Vayne, give Vayne side lanes. Vayne gets a double kill at 3 minutes, the game's over. Double kill vayne's bot lane, well she's still vayne, she can come back.
She takes over the game by being in the game. That makes her, i think without question, the most toxic champion in the game.