Vayne is ridiculous
Practically cliche' topic at this point I bet, but Vayne's kit is so difficult to play against and counter I have no idea how riot even thought this was balanced.
Problem 1: Her Q (Tumble) is basically a "screw you" to anyone with a skillshot. Most of the time the only way to beat an adc is with hard engage and crowd control, but most CC is skillshot oriented (Blitz Q, Thresh Q/E, Naut Q, pretty much anyone that isn't taric), its cooldown is extremely short at later ranks without even building CDR, and the attack damage boost is gigantic when compared with the mana cost.
Problem 2: Her W (Silver Bolts) is impossible to counter against. To counter most Marksmen, you build armor; but Vayne has true damage. So you build health to counter said true damage and then it makes it even more brutal because it has a max health scale. The only counter to true damage is health, but her w basically covers itself from countering.
Problem 3: Her E (Condemn) may be a bit awkward to get the stun right sometimes, but the problem is that outside of lane it's pretty easy to stun people, and even when you don't stun people you can use it to push engages and ganks back, basically giving vayne a free ticket home. But when she does land the stun, it lasts so long. Combining the stun and her W with even a little attack speed basically says "you're dead if I hit you with my E"
Problem 4: Her ult is such a ridiculous steroid, 100 mana is a decent cost, but the amount of stats she gets from just pressing a button and spending some mana is angering. She gets almost a fully stacked
worth of attack damage, invisibility on her "screw you" and her passive gives her 90 movespeed... considering that her Q and normal passive already make her difficult to escape from without a flash dash or speed boost (with still a slim chance of getting away) this just makes it so much more frustrating to play against.
Before you comment about how she's "balanced" take a close look at a vayne in game...(Also I know I pretty much ranted about her whole kit but the whole damn thing is so powerful)