I mostly play on Twisted Treeline, but my most effective build so far is:
Boots of Swiftness w/ Alacrity - I was off-tanking, so I needed to initiate and chase down a lot. With my Q lowering my MS, I needed the extra MS over my enemies to keep up enough to chase with it.
Frozen Mallet - Tankiness + CC
Black Cleaver - Extra tankiness, CDR for my abilities and E charges, and the ArPen.
Blade of the Ruined King - Extra damage, Life Steal for survivability, added damage to tanky targets (and everyone is tanky in 3s), and the active for added chase down ability.
Maw of Malmortius - Vi is an initiator that is always balls deep into team fights and gets focused hard because of it. So on top of the AP shield to help survive being focused, it adds quite a bit to her damage.
Zephyr - Extra MS, CDR, and AD is never a bad thing. Especially when you consider the extra AS will help keep her W procced in prolonged team fights, and the Tenacity removes the need for Merc Treads.