User blog comment:Krufix/A Scientific Discussion of Trollbuilds. Episode 1/@comment-89.74.37.222-20130226060130/@comment-122.2.31.136-20130227063322

Dude, AP Varus is totally different from AP Vayne in the sense that Vayne has no skills that scale on AP (so does Riven, Garen, Talon, etc.). Varus's Blighted Quiver HP% damage actually increases with AP, so does the on-hit effect.

Blighted Quiver: Chain of Corruption:
 * On-hit Damage: 10 / 14 / 18 / 22 / 26 (+ 25% AP)
 * Blight Damage per Stack: 2% / 2.75% / 3.5% / 4.25% / 5% (+ 2% per 100 AP)
 * Stacked Blight Damage: 6% / 8.25% / 10.5% / 12.75% / 15% (+ 6% per 100 AP)
 * Magic Damage: 150 / 250 / 350 (+ 100% AP)
 * Spread range: 550
 * Tendril break range: 600

source: http://leagueoflegends.wikia.com/wiki/Varus