I'm a Marksman main, currently in Master with around 200 LP. When i started on League of Legends, in mid Season 4, I had already been playing Dota 2 for quite a while. I reached 4200 Elo in Dota 2, when I chose to change to League of Legends. The leveling part of LoL, i took as my time to change my mind from Dota to LoL, the items, champions etc. So i played every champion in the game throughout my time from level 1-30. Mostly vs. bots, to learn the champions and find my mains, but later on vs. other players. I found Vayne really really interesting, and started playing her a lot. I started creating my personal builds for her, by taking impact in different enemy team compositions, and after now more than 2000 games with Vayne, I've come to a conclusion about wich build I find strongest and wich builds I find weak. NOTE: Don't FOLLOW this guide like it's a perfect way of playing Vayne, but use it as a guideline to find your own perfect build.

Let's start with the guide itself.

Enemy Team Compositions

I change my build depending on what kind of team I'm up against. Are they 4 squishy high-damage targets? A lot of CC? Do i need to instant burst them down? Do i need to poke? Do i need to tear down a high healthed tank? Okay.

Example:
The enemy team consists of a Taric, Caitlyn, Nidalee, Lux and Renekton. Nidalee is full AP, Lux too, and renekton is building as a bruiser.
In this matchup, there is 3 enemies with a high damage output, but low health. In this case I would build double CRIT/ASPEED + IE. NO Blade of the Ruined King because the faster I can kill an enemy champion, the less damage my team takes.

If I instead were up against a Caitlyn, Swain, Amumu, Renekton and Leona, I would build Phantom Dancer to kite better, Blade of the Ruined King for shredding their health and giving me more sustain, due to their high CC a Scimitar, and then either GA or Blood Thirster and a LW-upgrade as you prefer.