Every champion is good, and Vayne is fine, there's really not much you can build to stop her in the late game, and theres not a lot of other champions that can make the enemy team have to deal with something like that. Just because she's not flavor of the month or super popular doesn't mean she isn't good. There's people that get high elo just by playing AD Annie or Soraka, so you can definitely do fine as a real ADC with a lot of outplay potential.