Why play Vayne?
Vayne is a hyper carry and can absolutely wreck the enemy team in fights due to her true damage procs of Silver Bolts. Her ult and passive make her assassin-like, which allows her to dart in and out to kill and maim. Vayne is perfect for those who have mastered an easier Ranged AD Carry, like Miss Fortune or Ashe. However, Vayne requires superior positioning with Tumble to land great Condemns.

Vayne carrys and matches up very well late game with her damage and team fighting skills with a good amount of peeling and positioning.

A good example of a amazing Vayne players are people such as Royal Uzi, Doublelift and many other amazing players http://www.youtube.com/watch?v=gclBsjvcZ9s this is one of doublelifts amazing montages of insane plays.

PROS

-Very high high burst late and mid game
-Amazing true damage good for shredding anyway
-Amazing if played well with mechanics
-Fun and enjoyable champion
-Great for carrying
-Is one of the best duelists in the game
-Great for kiting

CONS
-Very short range
-Quite weak early game
-Harder to play in team fights however her damage is what brings her back

In my runes i take the flat armor seals in order to duel opposing ADC's and you will take less damage during duels and when getting harassed by enemy ADC's.
i take full flat AD marks for the extra damage while dueling and creeping. i take full flat MR glyphs in order to duel AP's and last longer in team fights. i take 1 movement quint for kiting and getting a ways and 2 quints of life steal for dueling and sustain in lane.
The movement quint can be replaced by many viable options such as a 3rd life steal quint or attack damage quint or whatever your preference is.
You can also switch out whatever you want in the rune page as this is all just my opinion on how i play her and how i would recommend you too play her too.

In Vayne's skills sequence it all depends on the situation.
If you need to get away or kite a melee try E'ing them away for escaping and if going aggressive try and E them into a wall for the stun. Tumble can be used for poke or can be used for mobility when running away from or chasing. Try and combo two silver bolts up then roll and shoot for the third silver bolt which can do reliable amounts of damage. Pop your ults and use your tumble to your advantage to escape or to chase or to break line of sight as it will make you invisible when your ult is popped.