I'm not highe Elo by any stretch, but I've usually found Trinity Force + The Lightbringer to be a decent build on her. Tri Force gives you some extra damage up-front and a decent sampling of AD, AS, Crit, and some MS and Slows for better kiting potential - which Vayne is reasonably good at, considering she can both knock back enemies and gap-close forward herself. Also, Dominion is over to fast to really get much out of going one Auto-attack attribute at a time, IE+PD builds tend to work in Summoner's Rift because you already have both scaling each other when things get interesting - so I like getting multiple scalers right away. Zeal and Phage offer a mini IE+PD interaction of AD, AS, and Crit - plus having some extra health to survive 100%->0% comboes is nice. Having the movement speed to outrun the generally faster melee champions can come in handy as well.
Originally Posted by BuddhaRice
I'm not a vayne expert or anything, but I'm pretty sure Xin > Vayne. Anybody with a gap closer is hell for vayne.
Vayne has a gap closer and Knocback herself though, which allows her to get some pretty good distance, and prevent most champions with gap closers from sticking to her that long. Only real problem with gap closers against her is if you can't tumble and/or Condemn someone directly after they use one on you - so if Xin is smart enough to save his third talon strike, you're going to have a bad time. If you can manage to Condemn Xin into a wall when Final Hour is available, he's toast - but being reasonably solid with landing 3-talon strike is a lot easier than lining up a stunning condemn.