Depends on the game and who you're going up against.
Starting off:
If you have a healer, take Doran's Blade. If not, boots and health pots.
Generally if you're fairly slow in starting off with Vayne, pick up 1-2 Doran's Blades (2 if you already had one at the start) and decide if you need sustain or damage. If you're getting kicked around in game, and this will depend largely on your laning partner, pick up a Vampiric Scepter, then get a Bloodthirster. Finish your boots after that, usually Berzerker Greaves.
After that, you'll want to judge the other team. If it's a lot of tanky, rely more on your Silver Bolts and go for a Phantom Dancer. Otherwise, grab something like an Infinity Edge or a Black Cleaver.
Late game, you're wanting to hit fast and hard. You should have at least one Phantom Dancer built at this point, 2-3 damage items, and a defensive item, selling the Doran's Blades as you go. It also depends on what you're fighting. If you're dueling a lot, grab another Bloodthirster so you out-lifesteal them. If you're in teamfights mainly and going for high priority targets, consider a Wit's End for the extra bit of damage, MR, and good attack speed.
Typically I run Flash and Cleanse on her, just because she's auto attack dependent. It allows you to get through hairy situations where you're hit with Exhaust immediately starting a fight.
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