Vayne is the single most fumbled champion design, and is in desperate need of a rework
As it says on the tin, Vayne is the original ridiculous. First off, there has never been a time where Vayne was alright. Vayne has either been the most oppressive unfun champion in the game, OR in the dumpster, and only played because literally no other marksmen was close to viable. There has never been a fun and interactive inbetween for Vayne.
So, lets discuss what Vayne's core concepts are, and what her points of interaction between other champions should be.
1.) Vayne is a low ranged champion. As such, it is fair to give her kit an abundance of tools which enable or overstack her damage, given that she will have to take large risks, and perform mechanically in order to make use of this. 2.) Vayne is a scaling champion. As such, it is fair to give her kit extremely powerful solo tools in the late game, given that she can not function independently in the early game. 3.) Vayne has access to windows of solo power, where she can function independently of her team, given that outside of those times, she must rely heavily on her team, not just to survive, but to get her to the next window.
Vayne fails on literally every single one of these, and historically has done so. So, lets do the list in order.
Vayne damage. Its high, its supposed to be high, and the largest enabling factor for this is her silver bolts passive on W, which is intended as a way to amplify her damage in a way that doesn't turn her into an assassin against squishy characters. True damage is, and has been, intended as a way to address tanks. However, this is not how vayne's passive actually performs in game. At levels 10-13 Vayne's W does around 10% of the targets max health. Without rageblade, thats 30 autoes, no matter what, which will definitely, 100% kill the target. No matter how much defense or health they build. And it increases by more than a third from there. Well 30 autos is a lot, maybe thats fairly reasonable. But hold on, AA -> Q -> E -> AA ->Q is 2 procs right off the bat. The opening combo for her is 20% targets max health, everytime, no matter no skillshots, no aiming, nothing special. Literally just beat your face on the keyboard, and its 20% max health guaranteed unmissable damage. Sure, thats something of a painful opener for her, but hey, at least she is in anyway vulnerable or can be punished for such an agressive usage of cooldowns. Eh, well not quite. With R, as well as Bork and Rageblade, just two items, Vayne 35% of max health, 20 of which is true damage, immediately. She does so during a rotation which does 535% Bonus AD, as well as 350 Flat damage minimum. First of all, if you can withstand the 1347 + 35% of your max health Physical damage, and also do enough damage to actually threaten or dispose of the Vayne your champion is OP. But, also note that during that time, Vayne has also healed for 300 health during that time. She has also stacked her rageblade, meaning that she will do 24% max health damage with Bork, as well as double the on hit effect of her Q, as well as her 11% max health true damage every other auto. That is more than a third of any champions health, plus 140% AD in just two autos, plus the two autos themselves, which later can crit. While healing for 12% of all that damage. Thats a lot of percentages.
So, to recap, Vayne not only gets more damage against higher health targets, but gains herself more effective health in the form of healing for hitting this target. All of this damage is burst out of stealth, and INCREASES after stacking rageblade, rather than falling off after such a strong opener. Meaning, engaging onto the Vayne is actually counterproductive to landing the pick, and going on her as the tank, makes it MORE difficult to kill her, by putting yourself in her range. Vayne actually is tankier, and harder to kill, the more she just facerolls. Her risk is lower, by just standing still in the front line. Additionally, her combo, when used on a squishy, will kill. Always, every time. Or, will get them so low, they can not remain in the fight. Because it does over 1300 Garunteed damage with two items, unmissable. As a level 13 Xin zhao with Cleaver, Warrior, and Tabis, that leaves me with about 900 Health after resists to play with. Minus, of course 36% of my max health for the 4 procs of Bork she got, so another 500 gone. I've got 400 health to play with. But wait, Billy Mays here tells me that double procing her W means I actually only have barely 50 Health remaining, because because I took 450 True damage, 22% of my max health. Keep in mind, that during this time, my entire combo has not actually hit the Vayne, because despite tumbling in, she has condemned me off her and out of my range. I can't even flash back onto her when the knockback has ended, because by the time she leaves stealth, I declare my auto, flash onto her, start the animation, and have it immediately cancelled, because she has already Q empowered autoed me, and re-stealthed. I'm no longer a champion. I have no cooldowns left, I have no way to fight the vayne, I can't hit her to restore my cooldowns, just 9 seconds of doing nothing. The Vayne has two options, she can either kill me, instantly, and hope that the other 4 players on her team haven't managed to completely spaghetti the fight such that they all were totally useless and died instantly. Or, she can totally ignore me, as I'm not a champion anymore, and just instantly kill my entire backline with 5-6 autos and Q, assuming the rest of her team did absolutely nothing at all, missed every ability, and was totally pointless. Less, if they weren't.
You would think that forcing the Vayne to use condemn, tumble back, and kite me out for a few seconds, would mean that Vayne wouldn't instantly be on top of my team, following every flash, dash, and blink both teams had used to start the fight. You would think that as a short ranged champion, she wold have in some way shape or form had to make a choice between going into and staying in the fight, or dispatching the dive who got to her with all of its cooldowns still up because her frontline is a bunch of monkeys. You'd be wrong though. With a 1 second cooldown 300 Unit dash, as well as 90 bonus movement speed the entire concept of being a short ranged champion is totally negated, because there is no window in which Vayne is anyones range, when they are not also in hers, and she has no difficulty getting to these targets whilst unimpeded, which she never is, because she is invisible and untargetable.
So, for "Having to take risks, and perform mechanically while in range of opponents", she gets a D- she can get to the fight any time she wants, and once shes there, shes untouchable.
Onto Vayne's early game. Its bad. Real bad. Vayne usually can't force anything against opponents, and struggles to really retaliate. Lucky for Vayne, she doesn't need to.
Mr Imaqtpie himself has complained about, and can attest that "if the enemy laner wants a no action lane, they'll get a no action lane" and Vayne is the queen of no action laning. If Vayne does literally nothing for the 6 levels, lets every wave push to her, just farms and never goes for trades, saves all or most of her mana to simply condemn away the enemy, she wins lane. If Vayne does nothing aggressive, at all, falls 30 CS behind, and loses 2 tower plates by level 7, doesn't matter, she won lane. If the vayne is not 0/4 and cowering from the dive at her tier two by 10 minutes, she won lane. I have seen movie scripts that are more ambiguous about what you're supposed to do, than how to play Vayne. Just afk the lane, buy boots and bork, back up out of range of the creep wave, and then use your passive to get back in time. Anytime the aggressive enemy goes on you, condemn them away, and tumble back. Have your support stand next to the wall, and condemn them into the wall if they try to engage on your support instead. Buy bork, and just sustain through all the damage the enemy does to you. Once Vayne has bork, and you can't kill her on the all in, you can't trade against her. She just heals to much off the wave. She has level 6, well hope you weren't looking to make the dive happen, because she has all the dashes stealth, knockback, and even stun if she plays it right to stop that dead in its tracks. To have a decisive win against Vayne, you need to be diving her on the tower before she has 6, and you need to be doing it with a champion who has lots of damage in lane, and a support who has ready access to CC and tankiness pre 6, and your jungler has to have burst and CC pre 6. Hope you enjoy playing Lee Sin Lucian Blitzcrank, and completely abandoning everything on the top side of the map. And hitting all your abilities against a champion with a dash on a 3 second cooldown. And I guess you just gotta pray that her support isn't something designed to just 2v1 the lane like brand. Or which totally negates all projectiles and hooks, like Braum.
Its completely unnaceptable that Vayne doesn't need to be in the drivers seat to win. Vayne should not be able to hold her own in a lane 2v2 until well after laning phase has ended. Vayne should not feel free to be any where other than on her tower for the first 15 minutes of the game, minimum. And while shes there, she should not feel like shes maximizing her potential.
There are A LOT of champions running around right now, where the easiest most consistent thing they can do, is also the single most effective way to use their kits. Thats a problem. Vayne is one of them.
"given that she can not function independently in the early game." D+ She can't function early, but doesn't need to either, essentially negating her downside.
Lastly, the windows in which you can abuse Vayne, largely when her ulti is down is nearly non existant. Want to poke her out? Well if you're Xerath and didn't hit everything and kill her, through her entire team then too bad, because as soon as the fight starts, she'll heal all of it back up.
Want to play an assassin and kill her, well she build defensive items, because her onhit build really only require two core items freeing her up to go defense. As a result, you can't 100-0, but she can 100-0 you, because she has so much guaranteed damage in her rotation, and can just choose not to fight. The onus is not on the Vayne to actually do anything, so she can just walk away and be confident that it will only be harder for the enemy to matter later, until eventually she 6 items and will just heal tank it all and 3 shot everything.
Meanwhile, while she makes it impossible to start the fight on her terms, while she has ulti, she can just go to a sidelane, and has prioirty over every champion. There are literally 0 champs that keep priority in a lane against Vayne with her ulti.
And even if you blow everything on the Vayne, manage to catch her, she spaghettis the play, tumble directly into the morg bind, and dies because she literally has no brain, even then, unless your team is literally inting, they just win the fight because of how many cooldowns Vayne demands, and how much health she will definitely take with her, assuming she doesn't actually take an entire champion or two down. If you have to have every champion dog pile onto the Vayne, and barely kill her, I got news for you, your ADC isn't living. And if your AD isn't there or isn't in the fight, well you have no cooldowns left to win the 4v4, and you essentially used everything while they used nothing to reach an equal numbers condition.
In summary, its too easy for Vayne to just do nothing, drag it out, hide, and wait, because the onus is never on her to actually do anything. No matter how the fight goes, if Vayne escapes, she'll win anything that happens after, and every ability in her kit gives her the ability to get out of the fight safely. As well as back into it if the tide ever turns.
"Windows where Vayne has to rely on her team to get her through to the next fight window" F- i'd give a lower score if I could. Her ulti gives way too much power for how weak she is without it, and how much usefulness defense she can manage to build to compensate for it.