Riot are planning some changes to ad carries and ad itemization after worlds; right now though any huge changes will likely be shelved. Vayne is strong atm in my opinion because she synergizes very well with BORK, tanks and juggernauts are very common, assassins aren't as common, hypercarries are on the rise and Vayne is the most self-sufficient of them, and all of Vayne's counters are in a worse position than her atm. Vayne herself isn't the problem, she'd barely been touched in the patches prior to her becoming common; it's just that this environment we're in atm favors her. Think about it, when we had lane bullies like Graves and Lucian on the rise, Vayne was nowhere to be seen for the most part.