Vayne's e should only apply a stack of silver bolts if you collide with terrain.
Title say it all - Having her E apply the stack regardless of whether or not you collide with terrain seriously muddles the distinction between good use of it and bad, and makes the auto-q-e combo far too potent for its level of counter play. Admittedly it puts her e on CD, but most of the time you can't capitalize on that especially considering she just knocked you away.
I'd also like to see her passive(and ult) MS be based off how directly you are moving towards your opponent, like Mordekaiser's w does on PBE; Rather then getting all the MS if she is moving anywhere in the 180 degree arc towards an enemy (e.g. most of the time) she'd get the full ms when moving directly toward her opponent, but only half as much if she was moving at a 45 degree angle, for example. Admittedly, this might be harder to implement for her as her passive would need to check for everyone on their team, rather than just one like on Morde.
These are both fairly small nerfs to Vayne, but both clarify elements of her kit that are for the most part hidden power.
(Posted on Reddit here)