The Combat Steps don't seem to specify when attack traits are triggered (should be in 4. Damage and Effects)
but they say something about the timing of "heal"-ing :
"Healing immediately removes an amount of damage from the target." (Rulebook v2, p.26)
Vampiric is decribed as a heal : "When this attack causes damage to a Living creature, the attacker heals (removes) up to half the damage caused (round up)."(Codex v1.0).
I guess that Damage and Vampiric Heal are calculated simultaneously, so the creature survives with 2 HP as there never is a failed 0 HP check in between effects.