Vayne
Looking at Vayne's Kit:
3 Hard Escape Mechanisms 2 Chase Mechanisms 1 Hard Stun 1 Burst Nuke 1 Scaling True Damage - Percentile (meaning you can have 400 armor and 10,000hp, she doesn't care) 1 Stealth Ranged ADC 1 Serious Power Boost
Throw Duskblade on top of this and she threeshots your Tank. The entire concept of "Don't focus the tank" doesn't apply to her, as she kills them as readily as she kills your carry. The arguement of "Well just CC her" works amazingly well if your team manages to get past her front line, through the support, and then your entire team manages to jump on her to ensure that she:
- Doesn't manage to simply condemn the tank who got to her and run away.
- Tumble into your team and wipe out the carries before she is CC'd.
- Kills whoever bought Oracles.
- Has a completely garbage team who can't peel for her.
- Didn't buy Guardian, because if she did she is killing the closest person to her.
- Gets zero CS, as she only needs two items to be effective all match.
- Never hits level 6.
So your entire team focuses her down, managing to kill the Vayne quickly... while having been ignoring their team entirely to pull this off. Who's APC, Tank, and Bruiser were beating on your carries. Their support was trying to keep Vayne alive, and despite that failure they simply default over to the Tank and APC, who now have the upper hand since Vayne knocked whoever came at her first to nearly dead.
P.S. Don't start reciting that 'Win Rate' crap at me. That encompasses the entirety of Ranked, and 85% of Ranked are garbage players.