I start off with a Dorans Blade on vayne and buy wriggles/boots of speed when ive farmed 2k after that u farm up until zeal buy the zeal and continue on whatever your doing finish off with dancer and transition into a tri-force from there you have all the items you really need. That point on you either build a bloodthrister or Infinity Edge or a Banshees Veil.

When your in lane with vayne you always want to be aggressive her damage is so good with sona or alistar. With this build you can also chase down anyone running away from you because of the phage slow and the dancer/tri/boots speed. It adds up to something around 500 and no normal champion can out run that.