Reworking Vayne
So 
... a character universally hated for a number of reasons. A character whose balance revolves more or less around a single item. A character who is probably a tad overloaded with free stats and a few too many tools to do her job.
I say tad and few because like I said, her balance revolves around a specific item's state and I think that probably isn't a good thing.
Vayne's oppressive strengths: -High mobility in a chase and great at kiting in a 1v1. -2 forms of non-action hard cc (knockback and stun) on 1 skill. One of which functions regardless. -An instant invisibility during Final Hour means she always gets initiative on her opponent without the presence of true sight. -70 AD steroid means she can rush attack speed, get behind, and still be relevant in fights. -flat and % true damage that makes her an even greater threat to low health targets than high health targets.
Vayne's glaring flaws -can't clear minions to save her life, sometimes literally. -must be within 550 range to deal any damage whatsoever. -must focus fire on 1 specific target to make use of her damage. -not good at fleeing from multiple pursuers. -Having high mobility is dependent directly upon her ability to fire off her tumble enhanced auto.
So what do we need to get her healthy? -A little bit of wave clear. -Less pure damage directly from attack speed. -A little bit of ultimate tweaking to make it easier to combat but also cooler.
Passive: unchanged except the interaction with her ultimate.
Q: Tumble Changes: Cooldown 9/8/7/6/5 mana cost from 30 to 20/25/30/35/40. No longer resets Vayne's auto attack timer or empowers her next attack. Instead, fires a bolt that strikes either the most recently attacked target or closest target within 700 units. Prioritizes champions if within the same range as minions. Deals 50/65/80/95/110% total AD and applies both on-hit and on-attack effects.
W: Silver Bolts Vayne zeroes in on a target as she hits it consecutively. If she lands 2 basic attacks or Condemn and an auto attack on an opponent, she readies a silver bolt to violently and painfully purge the wickedness from their heart. If she strikes her zeroed in opponent, they take 9/10/11/12/13% of their max hit points in true damage. This effect can only occur on the same target once every 12/11/10/9/8 seconds, giving an indicator like Yasuo or Udyr. Stunning an enemy with Condemn will reset the cooldown on a target. Minions take double damage from Silver bolts and splash the damage to other nearby (200 units) minions. Damage to monsters is capped at 400.
E: Condemn Vayne fires a very heavy Hextech Tetherbolt at her opponent. The weight of the bolt causes the target hit to falter becoming snared for .25 seconds. During this short snare, the bolt fires a grappling tether in the same direction it was travelling when it entered the target. If the target is near enough to solid terrain it embeds and very rapidly (think current condemn but even faster) drags and slams them into the wall, doubling the initial damage and stunning the target for 1/1.2/1.4/1.6/1.8 seconds.
R: Final Hour Vayne gains 20/35/50 Attack damage for 8/10/12 seconds and gains 25/50/75 attack range, this also similarly affects condemn and Tumble's ranges. When Vayne uses Tumble during Final Hour, she gains Shadow Step for 1.5 seconds ending if she attacks or uses a spell or item, summoner spells don't count. No longer buffs passive.
New Effect Shadow Step Semi-stealth semi-untargetability. During Shadow Step, a target turns shadowy black (think current Morde ghost), gains 15% movement speed, and cannot be targeted directly by attacks or targeted spells, but can be hit by AoE damage or damaging targeted abilities already in flight at 50% effectiveness. These effects can be completely negated by any true sight.
So let's review what we've done. Q no longer resets the auto attack but can't go on excessive cooldown and does a mini non-critting AA that goes beyond her standard attack range. It also costs more mana at higher ranks. W does -more- damage, but is prevented from rapidly affecting the same target consecutively and affects all targets equally. It also gives her a wave clear option. E no longer automatically knocks back, but stuns longer at higher ranks if she can get the stun. It also normalizes the effect of the skill when flashed during its flight. R now grants less AD but grants a bit of attack range making her team fight presence less polar. It also gives her some ability to survive bursty effects if there's no nearby true sight.
Keep in mind all given numbers from the ranges to the listed damages would be tweakable. It's more about making her more healthy as an AD carry while preserving her identity.
