As I think about it, Vayne has the only unclickable spell in game, Silver Bolts. The only similar thing i could think of is Cho'gath's Vorpal Spikes, but they can be toggled on-off. Vayne however can't turn off Silver Bolts.
So I thought that either Vayne should have a toggle on W or Cho'gath shouldn't on E. If Cho'gath can't turn off E, it's a slight nerf for him. First, he will often shove the lane when he shouldn't. Second, he may accidentally hit an enemy under his tower with a needle, drawing tower aggro and taking a shot. However, if Vayne can turn off W, it's a buff for her. Imagine her against e.g. Twitch near 2 fighting minions. When both are ~30% hp and Twitch goes into a lasthitting position, Vayne pokes him with AA Q AA, he has 2 silver bolts there. Then Vayne lasthits a minion and Twitch drops his stacks, denying Vayne from Q AAing him for massive damage from AA, Q and W at the same time. However, if she could turn W off for a lasthit, she could proc W on Twitch.
So how about equalizing Vayne and Cho'gath in their on-hits?