Vayne's Early Game
Right now Vayne is dominating the meta killing squishies and tanks and having incredible mobility and self peel. Vayne is supposed to be the late game hyper carry but she does just way too well at all stages of the game. So instead of just gutting her, or removing her true damage, etc. I think it'd be more fair to tune down her early game so it's not so easy for her to become the late game monster she is now.
A good start is reducing her base movement speed to 325 and her attack range to 525 cause with all her mobility she really doesn't need the extra range or speed.
Passive: Night Hunter Movement speed changed from flat 30 movement speed when running towards enemy champions to gaining 15/20/25/30 movement speed when moving towards enemies at levels 1/6/11/16. This change makes Vayne's early not so safe because she won't be as fast as she was chasing early and actually making her more late game oriented.
Q Tumble Mana cost changed to 50/45/40/35/30 from 30 at all ranks. This change is to make her early game less safe because it's mana cost is really negligible and is unnoticed early and late due to its super low cost.
W: Silver Bolts Silver Bolts now only apply stacks on auto attacks. This change is more to stop Vayne from getting a free stack with her condemn and stop the play style of her attacking twice then condemning to get the true damage off without fear of retaliation.
E Condemn Mana cost increased to 130 from 90. Cooldown changed to 32 at all levels but if you manage to stun the target the cool down is reduced to 16. This change is to stop Vayne from just being able to condemn and just walk away from them and stop any ganks from happening, higher mana costs mean she's going to have to watch her mana more often and the cooldown change is to make so if she does use it to push away people jumping on her she won't be able to use it again for a awhile.
R: Final Hour Mana cost increased to 100 from 80. Most Champions in the game have a 100 mana cost on their ultimate and Vayne's cost is fairly low for such a strong dueling ability so a little increase to even her out with others.
Vayne can be the super strong late game monster but she shouldn't just be able to win lane and destroy things super early and snowball so easily, and I think these changes can fix those problems Vayne has.