This is how you properly nerf Vayne while keeping her identity as a duelist.
Decrease her base Movespeed to 300
Increase Vayne's passive movement speed by 30 (Making it the same when moving towards enemy champions).
This would punish her for going in at the wrong time since she couldn't run easily. She has her E and ult for self peel if she gets caught 1v1 but this would at least let us lock her down in a teamfight.