If anything late game is where Vi shines. A full build Vi is monstrous. Don't build her for damage because her W takes care of that. Build her with tons of HP and grab something to increase attack speed (Wit's End or Trinity Force).
Personally I roll these full builds.
Jungle: Berserker's Greaves, Spirit of the Ancient Golem, Frozen Mallet, Randuin's Omen/Black Cleaver, Banshee's Veil, Guardian Angel
Top: Ninja Tabi/Mercury's Treads, Frozen Mallet, Randuin's Omen, Wit's End, Warmog's Armor/Black Cleaver, Guardian Angel
Tons of survivability (especially with her passive) and lots of damage output regardless because of her W. Obviously items are situational -- if you're snowballing top things like Black Cleaver and Trinity Force are recommended for pure damage output.