Vayne
What to Keep:
Her Q-Tumble is one of the most well known abilities and it is what gives her the trait of being so nimble. E-Condemn makes her so she has the outplay potential and a form of CC. And finally her Ultimate is a great way for her to show how to hunt the shadows.
What to Change:
P-Night Hunter: while keeping the movement speed part of her passive, I would also add a new component called In the Shadows where the closes enemy champ in range are Marked by the shadows. If Vayne hits someone with the Shadow Mark, she gets a burst of decaying movement speed.
W-Silver Bolts: is one of her strongest points in the kit, but I still dont understand how she does not have an Active; I'm pretty sure it is one of the few abilities that does not have an Active. I would add an Active where she would steal a portion of their movement speed hit by her next auto attack. Everytime she hits a Shadow's enemy, the cooldown of this ability decreases.