So I'm sure we are all aware of Vayne, The Night Hunter. While I do not believe she is "most OP champ ever nerf rito plz" this article will look at her in detail and look for potential balance changes that won't nerf her into oblivion.
TL;DR : Half Silver Bolts' %max health true damage but add a 80 BONUS AD = 1% true dmg ratio to keep Vayne's late game strong, and her early game weak. If necessary don't let Condemn proc Silver Bolts.
Passive: Night Hunter.
Only truly benefits Vayne when chasing over long distances or when she's tantalisingly close to attacking, mainly benefits her ult. The lack of ANY defense for Vayne in this passive keeps it fairly balanced.
Bonus mobility on a short cooldown is strong.
But i think Tumble is decently balanced. The scaling is weak early game since noone has any AD yet, and Vayne's first item BORK has low AD, and Tumble CANNOT pass over terrain, making it significantly worse than other dashes. (See: Graves, Lucian, Tristana, Kalista and Corki) Finally, if Vayne does not use the Tumble auto because shes running (and stopping to auto early game with low AS is suicide) then the ability has a 6 second static cooldown on top of its regular cooldown, again limiting Vayne's defensive options early until she has enough AS to use the Tumble auto to kite.
W: Silver Bolts
The main reason for Vayne being overly strong in my opinion. Silver Bolts rewards Vayne for trading, both quickly AND in extended fights, as Vayne has no limit on how many times she can proc the true dmg. However, this is also a major component of her late game, so nerfing the base % dmg OR removing the flat number should weaken Vayne's early trades enough to give her the weak early laning she should have. HOWEVER! Vayne's late game should remain as is to reward Vayne, so adding a bonus AD ratio to Silver Bolts will mean late game Vayne will see little to no change in dmg, while keeping her early game weak.
Balanced. A strong bit of self peel gives Vayne a much needed tool when the game isn't in her favour, and the fact that the stun is gated by the requirement for a wall gives it sufficient counterplay. The only thing I would address is the spell applying Silver Bolts. Top tier Vayne's will auto, Tumble reset auto into Condemn for an easy Silver Bolts proc, while its a mana heavy combo, it is a strong one, and removing the Condemn proc should keep her from getting easy true dmg Procs.
R: Final Hour
Balanced. While this spell comes with ALOT ( Flat Ad, Repeated Stealth and Movement Speed) the core strength of the ult is the kiting and skirmishing it gives Vayne, which requires smart play from Vayne to use Final Hour's effects to skirt around the fight and avoid getting caught, which can be countered with point and click CC (See: Maokai) and can be easily messed up (See: Bronze Vaynes) the high skill cap required to use Final Hour well keeps it balanced.