Hi guys! (and girls) This is my first Mobafire guide, hope you like it.
Background!!
My summoner name is PandaMedic and I currently play on the NA server at Gold 1. I've been playing since Season 1, where I mained top/mid. After going through a few ranked games, I gave up. At the start of Season 3, all I played were normals. I eventually played until my mmr was way higher than my ranked. This wasn't necessarily a good thing since most Golds don't trust silvers, and I ended up getting raged at most games. I started playing ranked again, and after a few agonizing losses because my adc couldn't carry, I decided to main adc. I'm also good at supporting and I have a few smurfs so if anyone wants to duo for some tips feel free to add me!

Pros

Getting her combo off does a lot of damage while not allowing much counter play
Does true damage
Scales incredibly into late game
Insane dueling, 1v2ing, 1v3ing, 1v4ing and 1v5ing potential
Underestimated in lower ELOs
Amazing kiting power
Very fun to play

Cons

Short range
Banned in higher ELOs
Can't do much without farm
"Countered" easily

Vayne has a little combo, which is executable because her Tumble resets your auto attack timer. This means (for those of you who don't know), you can Tumble right after you auto attack for a free auto attack. Her combo is this: Auto Attack - Tumble - Condemn. What this does is gets her 3 stacks off Silver Bolts while knocking them away so they can't trade with you. When you first start doing this, they will probably get 1-2 attacks off on you. Once you get good, they will probably only get a maximum of 1 attack off on you. This does drain a lot of mana, and her Condemn has a long cooldown, so be careful.

The choice between lifesteal quints and flat ad quints seems really one-sided. Doran's Blade has long gone stopped giving lifesteal, and having that +6 or +4% lifesteal is surprisingly insane in lane. All the ad carries I've seen have all gone lifesteal quints. The way I play Vayne, however, is quite aggressive in lane and with a good support I end up outdamaging their heal. Keep in mind that if your support has a heal, the lifesteal doesn't really matter, and it does help to have that +6.75 ad. Other than that, pretty standard runes.
Another choice in runes would be to take around 3-5 flat m-res glyphs, and the rest m-res/lvl glyphs. The reason for this is that most ap carries in the early don't do massive amounts of damage, they need to scale off some ap items first. By the time they start doing damage, your m-res/lvl glyphs have surpassed the m-res from the flat m-res glyphs.

For those of you who missed it, take Condemn level 2 if you're losing in lane/don't usually get 3 attacks off. This is pretty basic. If the enemy is playing very passively, and they don't have a healer you can get Tumble to level 2 early for more harass.

Don't take Cleanse. Worse comes to worse, get an early Quicksilver Sash. Barrier helps so much with in-lane trading, and late game the shield is better than an extra Quicksilver Sash that you will get anyway. Think of it this way. You can buy cleanse in game for 1550, but you can't buy barrier in game.

is "countered" by most people. Vayne is also the easiest adc to outplay with. If you're confident in your skills as her, pick her every game. If not, here are some guidelines.

This lane should be very Easy.

Dodge his Mystic Shot with Tumble and punish him when you can.

Skill dependent.

Vayne's short range makes it hard for her to trade because of Corki's Gatling Gun. Make sure to get your aa- Tumble- Condemn combo when he goes in to harass.

Skill dependent.

Double Up is annoying. Make sure to auto attack and then Tumble for the autoattack reset when she goes in to Double Up you. Otherwise, out farm her and you scale better

Skill dependent.

Graves can follow up your Tumble with his Quickdraw, so be careful about using Tumble to escape. If he Quickdraws towards you, knock him back with Condemn. Otherwise, try to outfarm him; Vayne is the best scaling ADC NA

Skill dependent.

Tristana is another other late game hyper carry. Her range also means she has an easier time in teamfights than you. Her Rocket Jump makes it hard to chase and escape, so try not to get behind on hp in the early game.

Not the easiest of lanes.

Since they changed Draven's passive, he has significantly reduced damage output. He still does tons of damage, however. If he trys to trade with you do your aa- Tumble- Condemn combo and make sure to Condemn him out from under his Spinning Axe.

Not the easiest of lanes.

Quinn is not picked very often, but when she is she is a pain when played right. When trading, make sure to dodge her Blinding Assault and don't be aggressive when you are targeted with Harrier. If her Vault is down for whatever reason make sure to get attacks off on her, but if not don't try to trade because she outdamages you early.

Not the easiest of lanes.

Don't underestimate Varus' damage output. Try to dodge his Piercing Arrow and Hail of Arrows with tumble, and watch out for his Blighted Quiver stacks.

Pray to god Caitlyn is bad.

While it is true Caitlyn is far worse than Vayne in the late game, if you are facing a Caitlyn, pray to whatever god you believe in she's a bad one. Caitlyn's range far surpasses Vayne, and although Vayne wins post-6, a good Caitlyn will keep you zoned out so you won't get 6. If you can, try to coordinate with your team to get a lane switch until you hit 6. If you can't, remember to not take unnecessary damage and try to keep your cs within 10 or 20 of Caitlyn's.

Remember that these are just guidelines! If you are good enough, you can win most lanes with Vayne.

Aqua Prison and Tidecaller's Blessing are godlike trading tools, and Ebb and Flow help if you somehow come out on bottom. Make sure to communicate with your support when you're going in.
His pull allows you to position for your Condemn