4 simple rules:
1. stay alive
2. farm safely
3. ask for ganks (if your jungler pressures the lane in your favour you can farm more easily and maybe even get a kill or assist)
4. you don't have to get ahead in your, you are one of the strongest lategame champions just farm if you can you dont have to trade or force turrets, Dragons or whatever

In the midgame you should have completed your botrk, boots and youmuus with These 3 items you are able to defend yourself and deal some nice dmg, so what you wanna do is fighting and trade kills, a vayne with 3 kills and 3 deaths will always win a 1v1 against a cait with 4 kills and 2 deaths, so eventho you might not always have the best stats you can still be a beast. The second Thing you want to do is pushing turrets to get to the lategame (to your strongest time) asap.

Everybody knows that vayne is a beast in the lategame, but why? because of her W which deals true dmg and % dmg to enemies. That means that a vayne can kill every tank without any probelm like no other adc(except for kog, he also got % dmg), and since you can peel for yourself with your E you can survive a lot longer than kog maw or Jinx who can only run and cry for help. Another Thing you can do with Vayne in the Mid and Lategame is splitpushing, because there arent many champions that can 1v1 a Vayne with a couple of items due to her kiting, dmg and selfpeel.

Cons:
-Weak Early
-Difficult to Play
-easy to bully
-Needs a good Support
-Low Hp

Pros:
-High Dmg
-Selfpeel
-% dmg
-true dmg
-strong mobility
-stealth
-one of the best duelists in the game
-strong splitpusher
-the most important thing, it makes fun playing her!