Hello my name is iSmurf, and this is my very first build ever, and its a Vayne build. I will appreciate any criticism, good or bad. Vayne is one of my favorite champions, and lets be honest one of the best in the game.

Pros.
Unstoppable late game
High DPS to any enemy
Great escape mechanism's
Possible Stun
Great farmer
Cons.
Slow early game
Very squishy early game

Okay lets start with the Marks (red)
The reason you should get flat AD is to enhance your farming, and high DPS early on is always a good thing
Now lets talk about the Seals (yellow)
The Reason you should go with armor per/ lv is so you have and extra boost in defense, especially needed as Vayne, you can also go with flat armor, but that's only if you don't feel comfortable with starting with low armor early on.
Lets continue unto the Glyphs (blue)
Attack speed is Vayne's #1 attribute, because her ability Silver Bolts is Vaynes primary source of damage, being true damage and it heavily rely's on her attack speed.
Finally we move unto Quintessence (gold)
Topping Vayne off with an extra bonus in AD is very useful in her farming capabilities, you can choose to get attack speed instead, but only if you feel comfortable with your farming abilities without the extra DPS.

The reason you should run 21/9/0 with Vayne or any carry for that matter is because it is your job to deal high DPS in a short period of time, and offensive mysteries only help you do it better, and 9 in defense is mostly to help you stay in lane longer to farm better.

Farming should be your #1 priority, but getting a kill in the process never hurts. Basically if you have 100-120 farm at 15 min's your doing well, 200 by 20 min's your doing great, and 300+ by end game you've done your job farming wise.

Starting off with Boots and 3 pots helps you stay in lane longer, meaning extra farm. The first time you back you should be carrying around 1.6 - 2k Gold so you can buy Wriggles Lantern and 1 Dorans' blade. The second time you back you should be carrying roughly the same amount and finish off your boots with Berserker Griefs, a 2nd Dorans' blade, and a Phage ( but if manage to go back with enough to finish a Recurve Bow as well go for it), by the time u have these items you should go for a turret kill, or already have it. Next step is to upgrade that Phage into a frozen mallet for its high HP count, and slowing proc. Wits End should be the next item on your list for its high attack speed and magic resistance. Then Madreds Bloodrazor for its Passive/attack damage/and attack speed. Then finish off with Ionic spark for its High attack speed and good amount of HP it gives. Selling wriggles for bloodthirster is optional but recommended, it gives a greater amount of attack damage and lifesteal, but you will potentially deal less damage to dragon and baron.

Vayne is a very fun and rather noob-friendly champion, and raping your enemy team with LoL's hyper carry should give you goose bumps every time, and remember to vote, comment and give me feedback on what I could of done, what i did wrong, and anything else you want to share with me and the other readers/viewers.