Haters gonna hate :P
Consider this. Vayne gives up quite a bit in terms of raw power when this is involved in her kit. For every instance of an advantage, something is compromised to make the champion, in a sense, non-overbearing. Vayne gets a lot of damage off of the W proc yes, but she can't distribute procs evenly like Gnar can; she loses all procs on a target if she switches, and the vast majority of her damage comes from simple physical damage. Only about 1/5 to maybe 1/4 comes from the true damage procs. So statistically, a lot of her damage is not linked to the W. Sure it hurts like hell, but it takes time to proc that, which you tend to not have a lot of when getting dove in full 5 vs 5 team fights. In smaller fights (2 vs 2 or 3 vs 2) Vayne have more time because less people are going to be attacking her, so she can get more procs off. That's more the area where Vayne is quite strong than full 5 vs. 5 fights. The reason I say this is because of how the champion is built.
Her passive gives bonus movespeed, sure, but it only works when chasing or moving towards enemies, never for running away. That movespeed is significant for clean up and chasing down stragglers, but it gives her nothing for getting away.
Vayne has a very short range Q dash (300 units, compared to 425 for Lucian E and Graves E or 600 on Corki W [1800 when the package is avaliable]) with a low cooldown that functions as an AA reset; fundamentally, this functions in no way as an escape, only an engage and a dodge. Sure you can tumble through a few walls, but your room for error is quite small, and in a full fight that room for error goes down a lot due to stress and the chaos of the fight.
Her E is nothing more than a push-back (470 units back), which is on quite a decent cooldown, even late game. This is primarily because she won't be leveling it except as her last maxing, which means that cooldown stays at 20 until she is at level 14 (CDR from items non-withstanding for the sake of avoiding build issues). With this in mind, other champions with dashes or blinks or hooks or MS buffs (Lucian, Graves, Irelia, Ekko, Tryndamere, Thresh, Nautilus, Leona, Garen, Nidalee, Kennen, Maokai, Zed is debatable but unlikely due to his W's duration counting with his cooldown, the list goes on) will have theirs up before Vayne's E is off of CD. The biggest thing it gives her is the stun if she land it, but that requires at least decent positioning by the Vayne, which is a 2-way interaction between her and the enemy champion; by that I mean you will never get pinned if you keep good positioning vs. a Vayne. Period.
Her ult gives her invisibility on her Q for 1 s, but pinks and Oracle's help to deal with that. In addition, the functionality of her Q doesn't change; it is still a short range repositioning dash and AA reset. Her ult also gives more chase potential but again, no getaway at all except for a 1 s invisibility. Even then, that won't make up for its short range. Part of the biggest reasons why Vayne can win fights is because she gets a reset along with the invisibility that negates damage on her for 1 second, unless she is caught in AOE or skill-shots (cough, cough).
Ultimately, Vayne's true damage is decent, but it doesn't make up even a third of the damage she deals in the average game. In addition, she has no escapes whatsoever, only engage tools. When she commits, she has to commit.
Biggest way to kill a Vayne is single-targeted CC (Lissandra, Maokai, Nautilus, Leona [her Q], Taric, Ryze, Alistar (once the combo is updated and it becomes much harder to mess up [hopefully] etc.). If a Vayne gets hit by any of these in a full fight where the team plays well, then she has no chance (QSS not withstanding, but QSS has a cooldown as well).
Just my thoughts. Hopefully that is sort of clear and concise to give another aspect of the discussion.
Sources:
www.champion.gg
www.leagueoflegends.wikia.com/wikia/[_] <-- champion used in discussion
I think the pinacle of this reached like -12 or so lol