Can a spell like Vampiric Touch drain life from a creature that is already at 0 HP?
I can't really be sure if the "no negative HP" rule means that the damage taken when in 0 HP does not "count" for Vampiric Touch if it's not "Massive Damage".
Example: A character uses Vampiric Touch on another character at 0 HP (with advantage and it's a critical). They roll for damage, which by chance, is not enough to be "Massive Damage" (the Max HP of the fallen creature). Does the caster heal himself, or is the effect wasted due to the "no negative HP" rule?