Please Read: How to balance vayne so she is an ADC and not an ASC
So lemme explain what I mean, Vayne in my opinion is not actually an ADC, she is a Attack Speed Carry that relies on true damage way to much. This is also why she is "overpowered", which i don't believe she is just hard to face when you don't know how. ( Buy these things 
) So how do we make vayne have a weak early game, and a strong late game like she is supposed to, and i don't mean after getting 
as lategame.
So simple solution to make vayne an actual adc, took me like 20 mins to think about at the library today between classes.
So what do we change, her Silver Bolts obviously. How do we change them TriTail? Like So
Silver Bolts: "Vayne's basic attacks and abilities mark her target with Silver Bolts for 3 seconds, stacking up to 3 times. The third stack consumes all Silver Bolts on the target to deal them true damage." So course they stay the same, what changes the scaling and how they work on 3rd hit
Upon the third stack of silver bolts, the target will take- TRUE DAMAGE: 20 / 30 / 40 / 50 / 60+ ( 10/20/30/40/50%) of the attack damage dealt on the third hit as true dmg.
What does this due, this makes vayne actually have to build damage for her kit to work and not just buy attack speed and rely so heavily on true damage that does %hp.Meaning she might have to or want to buy an 
first because as stated the % attack damage dealt on third hit, meaning crit would empower the true damage, but you would attack much slower, then you would have to build attack speed, but this allows for a weak early game because do you wanna max silver bolts when you have no dmg for it to work off of? Course not
This works blends in with her kit so well because * gasp* her Q-Tumble: Grants increased % attack damage on the next auto attack and R- Final Hour- Gives her a flat damage bonus when activated So maybe this dmg thing isn't so crazy sense you might have to tumble auto-attack someone for more true dmg instead of mindlessly building attack speed and right clicking. ( I know she is more mechanically difficult than that i'm just stating)
So how does this work better for vayne late game and many will ask " but now i cant melt tanks" well you still can, just much later
Lets do a scenario
The enemy tank is the Infamous 
with 
Your 
with 
Fight breaks out and your face to face with lets say roughly 4,500 hp Garen
You hit him with old silver bolts and 60+( 8% of garens max health) for +360= 420 true dmg
New Silver Bolts- at your build with Masteries and some dragons your at 350-360+final hour (+75) for a total of 425-435 ad give or take, you CRIT Garen with a tumble and the third proc of SB, dealing 435+50% for tumble X 2.5 crit bonus for 1,700 dmg before armor reduction, lets say Garen reduces the dmg on it by 36% after last whisper you now did around 1090 Attack damage and silver bolts did 50% of that -60+545 true dmg which is a total of 605 true dmg, which is about 13% of Garens max hp as extra true dmg.
HOLY MOLY that's a lot right, let me remind you that was also a crit+ult+Tumble last hit proc on SB. Plus this is only one scenario. Given the weak early game this gives vayne do to not having high attack dmg for her new silver bolts, she can be punshed, zoned etc. So if she does get to the point shown above your in some real trouble which is how it should be.
I am aware this would make her extremely strong late game vs squishies, well late game you shouldn't be A. separated from your team B. Trying to dual vayne solo unless your an assassin
Feedback Welcome, all scaling is just to get the idea across and can be changed
