TLDR: It's possible Vayne's too strong, so we're looking at her. 'Everyone says' doesn't necessarily equal true though.
There are a lot of anti Vayne threads on Boards, with you there. That indicates that, at the very least, a lot of players are finding Vayne really frustrating. Frustrating doesn't necessarily mean overpowered however, though the two are correlated. Champions sometimes generate a lot of frustration feedback even when balanced or underpowered (e.g. Blitzcrank really annoys some people regardless of his balance state and waves of anti Blitz threads crop up every so often). It's also important to remember that the population of Boards isn't necessarily representative of the entire player base (it's a subset of the english speaking population from certain servers).
Having said that though it's very possible Vayne's too strong at the moment. Her win rate in solo queue is quite high for such a popular champion. That, particularly when combined with the frustration she's causing, means she's worth investigating. Doesn't necessarily mean nerfing Vanye's the appropriate outcome though. It's possible we might find that Vayne's too dominant at present because the champions that should be keeping her in check in lane are weak and need to be buffed. Or because items she scales especially well with are overpowered. Or because other meta conditions (lots of HP stacking champs for example) are very favourable to her. Or even some combination of all those factors and too strong a kit, hence the need for some investigation."
In a reddit discussion concerning Meddler's Vayne comments, Scarizard elaborated:
I'd like to expand on this point, because i think Meddler's intent may be misunderstood; It's not so much that Vayne 'doesn't have a glaring weak point' - It's just that her weaknesses period aren't really manifesting.
Not every champion should be 'strong early, weak late' or 'weak early, strong late' (though this paradigm is often commonly seen among ADC's). However, 'okay all around but not amazing at any one specific thing' is how we end up with champions like Brand and Vel'Koz - they're fun and feel unique, but in the end are indistinguishable from a lineup of mages because they don't excel in a specific area (why not take someone that has a strength that stands out?)
Now, stepping further towards Vayne - 'okay all around, but not amazing at one specific thing' isn't true in her case. She's pretty top-tier at skirmishing and cleaning up, but her relative weaknesses (waveclear, sieging) clearly aren't presenting themselves at the moment. Reliability here simply means 'this champion's supposed to be a risky, but if you just pick it there's high likelihood you can take over the game anyway'. I can think of a few other champions (Gangplank, Fiora) that might be too reliable for how they're intended to play out atm, but that's my opinion.
I touched on a lot of different concepts here super lightly/quickly, so feel to hit me up if you wanna dive deeper. the deal w/ Vayne though is we're not exactly certain on what a changelist looks like for her (or if we're even truly going to do anything about her for 5.18, or all) so Vayne changes aren't set in stone. That said, from the conversations I've had if we were to change her we'd be looking at ways to detract from her high-end as little as possible and instead focus on 'what can the enemy do to react to her or stop her from going off?' this could present itself in a lot of ways, but like i said, still far out.
just wanted to drop by and add my perspective - i don't think we're against having some 'safe' picks generally, but well-rounded champion is a tricky (but not impossible) place to be as our roster expands
He continued:
"Let me preface this with this: no one is thinking of lowering Vayne's range. The general feeling is she's lost a lot of her risk factor - I'd agree that people pick vayne because she's fun - but i'd argue it's also because she's super reliable and strong; that said, we're not looking to make her disappear entirely. As for other marksman, there's a lot of stuff you'll see post-worlds focused on the role and its general power/satisfaction ratio, though i'm not really looped in to the specifics, nor would this be the avenue to preview those.
For the interim, we had buffs to Lucian and Twitch in 5.16 and we're currently planned for even more tuning to the 'lesser' marksmen in 5.18."
When asked if they still have plans to work on Marksmen, Scarizard shared:
"Simply put, yes.
The design teams as a whole have been looking for ways to make Marksmen more maintainable in the future on a number of different fronts - no ETA on when you'll see the beginnings of it, but it's a definite priority (though nothing before worlds).