Lets note that the vampiric trait gives you half the damage done as health and this card does not do that and as such is not subjected to the rules of that trait.
However, because of the vampiric sub type, I think it is a good idea to surmise that drain life can NOT heal more damage than what the creature has life, but from a literal reading on the card it can....... I think?
You roll 5 die of direct damage. You then heal the amount that the creature received.
So the question is does, a creature receive damage all at once or one point at a time?
Like I said I am going to guess that it was not intended to not let you heal 8 health from an amazing roll on a fox but I always assumed that if a creature takes 10 damage, you compares that to it's health, not add damage one point at a time and destroy the creature when the damage total is equal to it's health.