Vayne is not strong because of her true damage or mobility....it's because of her stealth.
Vayne is strong late game, but the real reason is not because of her true damage or mobility. Its because her stealth mode allows her to disengage and engage on her terms.
If you ever played against Vayne in stealth mode, you're at a disadvantage purely from the point of your reaction time. Once she enters stealth, you've lost your target. Once she reappears, she has re-positioned and gains the first attack. By the time you react and click, she has already landed her second or third attack. Even if you did not click anything else and wait for her to appear in hopes that you would automatically re-target her, it is still slow for the game AI to do so.
If Vayne does not enter stealth, she is almost at the same level as Lucian or Kalista, but she still has condemn for melee foes and potential stun for others.
What you guys think?