Headshots do not benefit from Vicious perk. I am testing equal Inherent Truth and Lazarus Bows and they hit the exact same when headshotting a dummy. When hitting in the chest and scoring a critical, the Lazarus Bow which has Vicious hits harder by roughly 10% than Inherent Truth, which does not have Vicious.
Penetrating Shot is not benefiting from Vicious Body shots, nor head shots on critical hits.
It does work for Rain of Arrows.