Last night my spouse stole my phone, so I didn't have a copy of the Codex or FAQ accessible... so I made a ruling based on how the game works in general: that is, that damage gets dealt to a creature, then you check to see if damage on that creature exceeds its life, and if so, it is destroyed and removed from play (or the mage loses, in the special case where the creature is a mage). In general, it would seem that the amount of damage on a creature can briefly exceed the life total of the creature before those values are compared and {destroyed/not destroyed} status of the creature gets updated.
So when asked how much damage a Vampiric creature recovers when dealing "overkill" damage (e.g. 4 damage dealt to a creature that only has 2 "hit points" = life minus damage remaining), I ruled that the Vampiric creature heals 2 damage, since 4 damage was dealt despite the creature only having enough life to survive the first 2 points of damage. And I was wrong, according to the Codex. But why is that? How does the Vampiric trait know how many "hit points" a creature has remaining (e.g. life - damage value)? Doesn't an attack simply deal damage, then "life - damage = hit points" gets calculated afterwards to see if the creature is destroyed yet? Or does damage get dealt "one at a time" and a creature never gets the opportunity to have more damage assigned than life?
P.S. The same should hypothetically apply to Drain Life and other effects that care about how much damage gets dealt.