Runes:
1. Marks: Greater Mark of Desolation (+1.66 Armor-Penetration)
2. Seals: Greater Seal of Vitality (+1.08 HP per Level / + 19.44 HP at Champion-Level 18)
3. Glyphs: Greater Glyph of Focus (-0.65% Cooldown)
4. Quintessences: Greater Quintessence of Desolation (+3.33 Armor-Penetration)
Items:
Starting Item: Doran's Blade
1. Boots (Mercury Treads in 95% of the games, due to enemy's CC. 5% Ninja Tabi if enemies are mostly AD. I NEVER went Berserker's Greaves with her: They're a huge waste imho, considering 35 tenacity from MT and the early MRes you get.)
2. Trinity Force (Starting with Sheen for Burst-Dmg, then Phage for HP/Dmg, then the Zeal. Trinity offers early- to mid-game Burst and survivability, due to more HP, movement-speed and Q procing with Sheen.)
3. Madred's Bloodrazor (Starting with Madred's Razor, then Recurve Bow, since it decreases the time you need for 3 hits, finishing with Pickaxe.)
4. Frozen Mallet (Offers even more HP and adds a little bit of damage. Furthermore its passive synergises with Vayne's chasing ability.)
[Depending on whether AD- or AP-carry of the enemy team is fed and/or seems a threat to you, i get these 2 in no particular order]
5. Banshee's Veil (What can i say? More HP, more MRes, can block critical spells. If you can't afford it at once, i advise you to go for Negatron Cloak first.)
6. Atma's Impaler (With Banshee's Veil, Frozen Mallet, Trinity Force AND your HP/LvL-Runes this item is a MUST-HAVE on Vayne imho, since it offers +45 Armor, +18% Chance for Crits and over 70 [!!!] Attack-Damage ... for 2355 Gold.)
Situational Items:
1. Hexdrinker (I get this right after Sheen and before finishing Boots, when I either sololane against Champs like Vlad/Annie/Veigar/etc., OR if there's a Karthus in the enemy team.)
2. Last Whisper (I only get this, if I find my enemy team's Carries are intelligent enough to actually buy some kind of armor. Otherwise it's just wasted money imho. If the enemy Ashe has 50 Armor, is it really worth buying it?)