603.6d Normally, objects that exist immediately after an event are checked to see if the event matched any trigger conditions. Continuous effects that exist at that time are used to determine what the trigger conditions are and what the objects involved in the event look like. However, some triggered abilities must be treated specially. Leaves-the-battlefield abilities, abilities that trigger when a permanent phases out, abilities that trigger when an object that all players can see is put into a hand or library, abilities that trigger specifically when an object becomes unattached, abilities that trigger when a player loses control of an object, and abilities that trigger when a player planeswalks away from a plane will trigger based on their existence, and the appearance of objects, prior to the event rather than afterward. The game has to “look back in time” to determine if these abilities trigger.

Example: Two creatures are on the battlefield along with an artifact that has the ability “Whenever a creature dies, you gain 1 life.” Someone plays a spell that destroys all artifacts, creatures, and enchantments. The artifact’s ability triggers twice, even though the artifact goes to its owner’s graveyard at the same time as the creatures.

Ah, I thought they might not be dying "at the same time" because the Avatar wouldn't die until the player's life has been reduced below 6, which meant damage was already done, which meant the Vengeance was already dead. Sort of a second-pass "whoops Serra Avatar, we missed you the first time around, now get to the graveyard you silly goose".