So once a Vayne gets a lead...
It's perfectly reasonable for nobody to be able to do anything besides pray that they are bad or bring the entire team just to temporarily shut her down? (or Ekko, Leblanc, Kat, etc)
Once these champs get a lead, they're going to get gold faster than anyone on the map. Even the people on your team who were doing well can't compare to the snowball, and the strategy of that game almost entirely becomes focused on that one champ. One guy in a 5 man team becomes so important that everything you do has to be done around them. That doesn't sound like a team to me.
I know you inevitable downvoters like playing the game like it was team deathmatch, so you want this to be a thing. I have an idea though, since this game isn't designed to be team deathmatch, and other games are, maybe go play one of them? This game is designed to be a team game, played with 5 people working together as a unit to take objectives. When you have some people trying to work as a unit, and some just ignoring everyone and everything and doing whatever they want, one or likely both groups is not going to have a good time.
If these champs need to remain how they are, why not at least cap off how fed they can be? I mean what kind of nonsense is a level 18 max build LB running around in a game where everyone else is 12-14. What's the point of that? One guy runs around killing everyone instantly and nobody else matters for the rest of the game. High level play will never see a champ do this, and that's what the game is balanced for. So why not put in mechanics that would stop this nonsense?
it is one of the few lanes with multiple hard CCs. Starting at lvl. 3 Vayne is deadly if even a single CC lands on you or your support, since she can follow up with her own stun.

<-MFW I ran into almost this exact comp and had to try and carry as 

Active, she relies on autoattacks to deal damage.
A late game
Helps immensely.