Reduce the movespeed bonus from Night Hunter
Yes, I'm suggesting a Vayne nerf. Hear me out for a second?
So, people are starting to acknowledge that Vayne has become a problem. Many people attribute this to the "tank meta", but I feel as though this is only part of the problem.
When we hear complaints about Vayne, most of them are associated with the %max hp true damage from her Silver Bolts (W). Personally, I think this should have never existed, but this passive ability only augments what I believe is the real problem: Night Hunter.
Vayne is identified by her ability to assassinate single targets extremely quickly. She has always done that extremely well and I think that Night Hunter is what allows her to do this so effectively. For those of you unfamiliar with the exact numbers, here is the tooltip for Vayne's passive:
Vayne gains 30 flat movement speed when moving towards a visible enemy champion. This is boosted to 90 flat movement speed when Vayne's ultimate ability -- Final Hour -- is active.
So why do I think this is problematic? Well, let's take a look at a few items that have been either changed or introduced to the League:
This item is Vayne's bread and butter because of its passive and active abilities. It has also received significant buffs that have consequentially made Vayne stronger.
This item was already a significant item for supports because it gave them every stat they would need to be the "Batman's Belt" their team needs to turn teamfights in their favor. This item was also recently buffed into a more favorable state.
Oh lord, this item. It needs no description.
If you haven't found a pattern yet, these items all have active abilities that grant movement speed. These items are also ridiculously common and it isn't unlikely that you're seeing at least one of these items built in every game. All of these items give Vayne even more tools to stick to a target or peel herself off of a threat. But here's the thing, though:
She already had enough to begin with.
Vayne is one of the few ADCs in the game that can eliminate unfavorable situations for herself through the use of Tumble (Q) and Condemn (E). We saw a similar scenario with Tristana in season 4 where she was extremely safe and had two ways to peel herself off of people who wanted to get in her face. While Vayne doesn't have resets, she does have the ability to reposition herself every two seconds with a rank 5 tumble and can push divers away with condemn. Aside from this, because a lot of the items that give critical strike and attack speed -- which is what Vayne is encouraged to build -- also give movespeed. Vayne will likely be outrunning every enemy with Night Hunter active before we even consider the tripled effect from the ult. There are very few ADCs that could even rival Vayne's kiting ability to begin with. With tanks back in season, it's now much harder to get to Vayne when she's safely behind a 5k hp wall of meat or two.
In conclusion, I believe the shifts in the meta along with the item changes I described are creating situations where it's extremely hard to exploit the weaknesses in Vayne's kit. I feel that these items are healthy for the game overall and don't need any changes beyond the ones they've received (like the much-needed nerf to Righteous Glory). With these indirect buffs to Vayne, something has to be toned down so that she's less problematic.
TL;DR: Vayne's too speedy.