Theres multiple things you need to note when picking Vayne as your ADC in Draft Mode :
-Your Support, who is it? Leona, Blitzcrank, Sona?: Vayne needs a support that can protect her well. Leona, Janna are great with Vayne because they can protect very well. Sona makes it so that your lane can poke a lot early game so you dont get harassed out of lane while you farm and get sustained. Leona gives you the CC you need to do tons of damage at level 6,and with good coordination you can chain CC pre and post 6. Janna gives you a shield, and can disengage and protect very well, especially vs aggressive lanes, which can turn the tide a lot of the times. (I dont talk about BC,because he should be banned every game)
-Who is the enemy support? Soraka? Sona? Leona? Taric? There are some supports that will make early game domination of Vayne very easy, mainly, Taric, Leona and Sona, usually, if you see these, you DONT want to pick Vayne as your ADC. Taric can stun you, and let the enemy ADC, intiate on you, works best with Graves, Ezreal, etc. And easily either kill you, or make you back out of lane. Leona can get you killed very easily, you have tumble, you can dodge her E, and you can condemn her away, but its a risky business I wouldnt suggest picking Vayne vs a Leona unless you are really, really confident in your skills, because pre and post 6, she can lock you down while her ADC insta-gibs you. And Sona can just poke you out of lane easily early game.
-Who is the enemy ADC? Caitlyn? Graves? Ezreal? Caitlyn should beat you early game, but if you play it safe and run back every time she runs forward you can dodge alot of the harass. Sacrafice farm if you need to, dont feed and you should beat her easily post 6. Graves,depending on his playstyle, just try to farm, and if you have a aggressive support you can go for a kill pre and post 6, just bait out his Q before an engage and try to chain cc him and you should do well. After 6 if youre both even, try to tumble dodge both his skills shots, his Q, and Ult, and that should secure an easy kill. Ezreal, at level 6 you have roughly the same damage output. Early on, Ezreal can of course kill you if you decide to engage him, but post 6, your damage output should be much higher. Key thing to killing Ezreal in lane as Vayne is using minions to your advantage, and making him hit them with skill shots while you get Silver Bolts proc's off on him. Should always tumble to dodge his skill shots, if hes in your line of sight and can free cast into you.
-What is your composition? Do you have a Nautilus jungle? Or Malphite? Jax? : Vayne needs a strong front line, and people to peel off her, so she can go rampant and kill everything in sight. If the enemy doesnt focus you, and youre evenly farmed, you should win every team fight. Always wait for the enemy to engage on your frontline before you join in, otherwise theyre probably waiting for you to get in range so they can focus and kill you quickly.
Item wise: You always want to get a vamp scepter as soon as possible, either before, or after the two Dorans blades. Depends on how the lane goes, if youre getting harassed a lot, rush Vamp Scepter. If youre just farming and doing ok, go for the Dorans, and then Vamp scepter. After that Berserker greaves.
And yes, your first major buy should be a Phantom Dancer. Max your W. 1 point in Q and E. Get your ult at 6,11,18. Your ult gives you enough AD for early game to makeup for the AD loss, and the amazing amounts of ASP and Crit you get from PD, coupled with the free MS and ignoring unit collision, makes Silver Bolts amazing. W is your natural damage steroid, you always build around your damage steroid. After PD, you can either go to IE, or Bloodthirster. IE and PD will give the most damage. Bloodthirster is good if youre ahead, and you know you will be getting focused. After those either BT or IE are built, build a second PD.
Remember, ASP is still your biggest damage booster. After that, you can go for a LW or a Guardians Angel because your damage should be really good at this point and if you either need to more Penetration to kill armor stackers, or more surviability, most of the time you will go GA as damage tends to not be a problem on Vayne.
Sorry if its a bit jumbled and such, you can always add me in the game and message me with any questions. My favorite ADC's are Ezreal and Vayne.