This is an unconventional guide for an unconventional pick, Vayne top lane vs. Darius. Vayne wreaks havoc on Darius for a few reasons.

• Vayne is ranged. Darius says: "hurr hurr I'm a melee bruiser who can't deal with any champs that can kite my e"
• Vayne has % health true damage. Darius says: "brb crying in fetal position"
• Vayne top allows you to run two AD Carries on one team, meaning you destroy teamfights. Darius says "wats a teamfight i get all the kills my team is bad".

Pretty standard setup.

Options:

Attack speed is the lifeblood of Vayne's destruction, allowing her to proc her true damage more...

... but you might be better off with MR in teamfights, in case one of those pesky AP Carries is fed.

You can also take flat MR, to mitigate damage from Hemorrhage.

Get these. Always. Start Boots and 4x Health Potion. (Recall at 1:20 for 4th pot)

Wriggle's Lantern helps you last hit and gives you some armor to help with Darius' physical damage.

Bloodthirster is the preferred lifesteal item for an ADC. Unless you're getting crushed by a camping jungler (wriggle's) or the rest of their team is composed of Kayle, Dr. Mundo, Cho'Gath, and Shen (BotRK for even MORE % health damage), get BT.

Blade of the Ruined King synergizes incredibly well with Silver Bolts against tanky comps.

Tumble resets your autoattack. This means that you can last hit two minions that are about to die nearly simultaneously. Use this to your advantage. Last hitting is more important than kills, four waves of minions (two minutes worth) are worth more than first blood.

This is why Vayne crushes darius so hard. Proc silver bolts on darius and he'll back the hell up in a hurry. After you get rid of minion aggro, stand between him and his minions, preventing him from getting gold and experience. if he refuses to back off, proc silver bolts again until he does. If he still doesn't back off, just kill him.
If he starts walking towards you, and he hasn't used his pull in a while, move towards him, auto him once, then Tumble away from him. You are now free to farm and harass him for 15-20 seconds.

Vayne beats Darius. A starved Darius is a losing Darius. A losing Darius is, well, losing. Go kick some Noxian soldier a**.