I think the only real issue with Vayne's E is that it can stun you for longer than 1.5 seconds.

Because of the way the knockback works she can effectively stun you for 2 seconds, an extra .5. The problem comes when it actually punishes you for being farther away from the wall, until you are far enough away that it won't stun you. Her knockback ends after the duration ends or you hit a wall. That means if you are directly next to a wall you will be stunned for less time than if you are just far enough away to still hit the wall.