Vayne or Corki

Depends, if your in derpelo and are good at adc and just bad at the game, Vayne is going to be better nearly 100% of the time as the games go long and teams lack coordination and vayne does very well late game even if you just right click on someone and close your eyes you can be 0-12-2 and suddenly 38-40 minutes rolls around and you can just kill everything you click on if they are bad.

She is also very good in high elo but relies a lot on how good her support is and if her team actually wants to peel in the higher tiers.

Corki is really only good with the right support and if your team understands a power spike and wants to act on it. Yeah he's got nice wave clear and can farm far more safely than Vayne which is good if you have a more mid game oriented team and you are on relativity equal skill footing against your enemy lane.

Least, that's how I see it.