So when you start off i would Start 4 heal pots and 1 boots of speed ! reason i wound'int take long sword and 2 heal pots is because your runes have enough damage to hold you till you go back and buy. When you go always start "Q" first then your "E" You are probably thinking you should get your "W" before "E" but you always want to play it safe in case of a gank comes bottom lane!

Whenever you go back i would pick up 1 long sword,Upgraded boots and 3 pots if you don't got enough; then you might just going have to go back sometime later in game. In mid game you should always rush Blade Of The Ruin King make sure that you use its active during fight cause on Vayne it deals a lot of damage!

This is when Vayne shines. Damage late game is one of the hightest damage among the AD carry. You are the main damage of your team. You must have your two big AD Items and building your [items/last-whisper.png]

Nashor must be warded EVERYTIME. If they go drake GO NASHOR. Nashor is more important than drake late game. Stick with your team, push the towers even with your short range. Be with your team if they go nash or drake anyway you have to stay with your team everytime late.
Push lanes and tower.
Teamfights are crucial to win the game.

Armor Pen : In the current meta almost everyone get yellow armor runes, red armor pen helps you a lot to deal more damage early game (against champ). Better if you aim mid/late game.
AD : AD runes let you last hit minions caster under tower (after 1 tower hit) and scale very well with your E and Q, you may deal less damage on your opponent but the damage output from spells is better. Synergies with LS Quints. Scales with BotRK active.

Armor : Great early game, but less and less efficient throughout the game (especially when the enemy AD carry gets Last Whisper). At the beginning of the game, you won't take true damage (most of people gets armor pen marks), letting you a great lane sustain. Plus armor runes are very cheap (one of the best cost/efficiency in the game), it's a must have.
Strength : I used to take these runes, but there are not that good anymore. Certainly you will have more damage : ONLY ON MINIONS. If your opponent is using armor runes while you use AD runes, you will hurt him a bit, but he will hurt you HARDER. Since people use armor runes on AD carry, strength are not efficient. You might use them if you lane with Taric (free armor).
Mana/Lvl : Vayne is not mana reliant, except if you spam your E but you don't have to spam it. It is a big waste to take mana/lvl instead of armor, you will lack of survability.

just go 21/9/0 should go prefect!