Honestly, Vayne is problematic and its purely because her kit is pretty toxic. She's a late game champion but her level 6 bot lane is probably one of, if not the strongest in the game. Like, I have a really hard time thinking of any ADCs that get as big a boon from their level 6 as Vayne does. Maybe Tristana because that's the point at which she can start assassinating her laner solo but even then, Vayne's does grossly out shine her's because Vayne, if she's even moderately good, completely destroys even Tristana at level 6. And this is while scaling better than Tristana does; stealth, massive AD steroids, low CD mobility, true % health damage, decent team fight, amazing 1v1, relatively low CD knock back and situational stun. A really good Vayne undeniably out scales every ADC in the game. So why does she also get an amazing mid game power spike with her ultimate?
It honestly largely comes down to her tumble gives her way too much damage(the steroid on it is absurd and provides her with way too much burst for an ADC that's intended for tank shredding; Tristana gets a lot of flack for being bursty but that's the point of her and she trades it off by not being the best until really late at dealing with tanks. Not Vayne though. Vayne gets to do both) and the fact that when they reworked stealth, they removed a huge part of the counterplay to Vayne for other ADCs. Before, when Vayne hit level 6, you could reasonably have you or your support toss down a control ward and at the very least, you could retaliate her damage. Now you're just forced to flail helplessly around hoping for a chance to return an AA or you have to burn a lot to try and get away from her and wait out the ult. Which is hard because her tumble CD is stupid low and her passive gives her bonus movespeed toward enemy champions, which by the way, doesn't make sense to me. :/ Other short range ADCs specifically have the problem of chasing as one of their weaknesses; I'm looking at an un ulting Twitch, at Xayah, at Sivir. At least two of those, if they want to chase, have to actually hit their opponent for their MS. Not Vayne, she just passively gets it for free.
She really needs to be reworked into something that's less overloaded and toxic for the game. Something needs to be taken away; the low CD tumble, the AD steroid it gives, the stealth duration, the control ward immunity her stealth has, the true damage... Riot needs to take a step back and look at Vayne and decide wtf is she supposed to be? If its a tank and bruiser shredder, take away her ad steroids, put more power into her true damage and give her a slightly longer range(though maybe not since she already has more range than several other marksmen; level 1 Tristana, minigun Jinx, Kai'sa, Kalista, Xayah, Sivir, Lucian, just to name a few) so that her teamfight is better. If you want her to be a duelist though, the true damage needs to go and something needs to be worked out so that she has some form of counter play for everyone else around her. She does not need to be both the best tank shredder and the best duelist and the best late game ADC. Something needs to give.