Each time you take damage, you gain 5% of the damage taken as attack power, up to a maximum of 10% of your health.
Patch 4.3.2 (31-Jan-2012): Vengeance is no longer triggered by receiving damage from other players. Patch 4.3.0 (29-Nov-2011): Vengeance has been redesigned slightly. It no longer ramps up slowly at the beginning of a fight. Instead, the first melee attack taken by the tank generates Vengeance equal to 33% of the damage taken by that attack. In addition, as it updates periodically during the fight, it's always set to at least 33% of the damage taken by the tank in the last 2 seconds. It still climbs from that point at the rate it did previously, still decays when damage is not taken, and still cannot exceed a maximum based on the health and Stamina of the tank.