If Vayne had the same hitbox on her stun as Poppy...

Her win rate would fall 10%.

Maybe it's not that drastic, but after playing Poppy quite a bit on the PBE, I've noticed it's possible to miss the stun even when there's a wall roughly in the right direction because your angle was off. Vayne's is foolproof.

Poppy's stun feels right, hard to pull off but rewarding when you do. Vayne's feels like bullshit that can't fail so as long as you have a general feel of how it works.