Grain of Salt 1: High skill cap champions (Lee and Vayne)
It's hard to cater to the weak invisibility of Twitch when pink wards could be invisible and you needed other pinks to fight vision wards without inherently buffing the duelist Vayne, It's hard to hit the sustain of Trailblazer without buffing those sustain junglers with relatively quick clears (
) , and moreover the flak that the balance team gets for not taking care of each and every champion on the RIft.
First and foremost, the power of these champions receiving small buffs seemed exponential, why?
Simple, The champions didn't go from level 50 to level 100, they went from level 50 to level 60 while all the other champion dropped from 4050 down to 1035, giving the illusion that they are buffed. High skill cap champions work like this 
Play poorly - give off little of max potential (Let's say 10% of max potential that playing perfectly would give) 100 damage
Play well - give off as much of max potential (Let's say 80%) 800 damage
That's a 700 damage disparity, but what about low skill cap champions? 
Play poorly - give off little of max potential = 10 damage
Play well - give off as much of max potential = 400 damage
Yes, harder champions give off better rewards, but shouldn't playing poorly give off the same rewards instead of giving an obvious advantage to the
who can't kite or the
who can't combo properly?
Here's my view on how to reduce their poor play down to the 10 damage equivalent to Annie's
Let's look at why these two champions in question are so strong, Lee Sin's Mobility and the incredible damage Vayne gives off mid to late game."
Does this mean we need to nerf Lee Sin's mobility? Not necessarily, we can hit the other areas of his play - such as his ability to smack you in the face instantly.
Change to Lee Sin: Your attacks and spells land a stack of Handicap on the enemies, attacks land 1 stack and spells land 2. You must have two stacks of handicap in order to use your ultimate, For each stack of your Handicap, your auto attacks will gain a bonus damage.
Change to Lee Sin: Your first spells or auto attacks deal a decreased amount of damage on a target without 3 Handicap Stacks. (Changes Q R Q to Q Auto R Q)
Change to Lee Sin: Your ultimate consumes all Handicap Stacks.
Multiple handicap stacks do not increase stats or offer anything more than the increase of duration that it lasts (1 stack = 2 sec 2 stack = 2.5 sec etc) This means that his overall damage is reduced, what is the compensation?
Change to Lee Sin: Allies attacking champions with Handicap stacks promote defensive growth, for each unique champion attacked Lee Sin gains a portion of his bonus armor and magic resist.
Change to Lee Sin: Champions that perish under the effect of Handicap increase Lee Sin's health regeneration and give him a flat AD boost and defensive stat boost for a few seconds.
Change to Lee Sin: Safeguarding your allies transfer/donate a portion of your defensive stats.
Attempting to shift Lee Sin's playstyle into an Assassin early and mid game to Utility late to Assassin early and tank Mid/late will balance out the immense raw power he provides in dueling situations. Monks are not fighters, they are defenders. His new W gives him option to peel alongside his E but makes the instant disengage from his ultimate impossible, allowing assassins a few seconds before they are punished for overstaying.
What about Vayne? How can we nerf her damage from mid to late? You don't necessarily have to nerf her damage. Here's a very simple but effective change to Vayne's kit.
Change to Vayne : Vayne's bolts now deal current health true damage, her ultimate causes Vayne to uses special bolts saved especially for Demons and Witches from her unique crossbow Nightwatch that changes her damage to true max health % damage.
Change to Vayne : A good vayne can be distinguished from a great vayne through her condemn, so how can we drop that poor play potential?
Vayne's condemn will only apply silver bolts if it collides with terrain, it will apply one stack and no longer deal double damage upon collision.
Change to Vayne: Her ultimate Final Hour stealths Vayne for half of the cooldown on Tumble, maximum stealth duration of 2 seconds.
Change to Vayne: Vayne's Tumble auto attack empowerment only lasts for 3 seconds instead of 7.
Changing Vayne from a mindless cannon into a champion that can now flexibly choose between mobility or damage.
This keeps the core part of Vayne's kit while reducing her dueling potential while her ultimate is down, offering a nerf to her true damage and providing an alternative maxing route from her basic Q max. Maxing W now boosts mid game damage while increasing stealth duration while maxing Q boosts early game mobility and damage in return for shorter stealth duration and weaker mid game damage.