Vayne needs this

First let's look at her history a bit Back in S2 she had a very hard early game, because a lot of the other ADC were much stronger than they are now, sustain was harder to come by, and she had to chose between AS or damage/lifesteal

Now other ADC have been nerfed, her sustain by mana is better with the mana changes that went around in S3-4, and she got BotRK, an item that offers her sustain, damage, and AS, this item in itself have her an item she could rush to give her comparable damage spoke to IE, fixes her weakness of sustain, and have a powerful on active item that can be used to run or chase or turn a fight

So she is much stronger now that she was before for early game

History over, this is all that she needs

A nerf to her base mana regen, with a buff to her mana regen growth What does this do? It reduces her safety in lane through less tumble spam, resulting in her having to play safe, instead of always relying on tumble to fix a positioning mistake in lane

Yes it's a short range dash, but it's enough to dodge a hook or pull that would have otherwise hit, but as is its spammable to the point she doesn't have to care if you have a blitz zoning, tumble the hook, keep CSing, if blots tries to run in then she walks away, if tumble is t already off Cd for her to be aggressive and get her silver bolts proc while moving away at the same time with tumble Spam

That's all she needs, to have less spammable tumble at early levels