"explain drain life then."
Vampire heals through vampiric damage which is also physical damage.
Drain lifes heals through spell damage.
Spell damage, when given to a creature, gets rid of previous forms of damage.
Its not the fact that spell damage can't heal the owner and damage an opponent, but SoW getting rid of previous forms of damage. None of this is made up for this specific situation either because you can confirm what SoW does with poison creatures and the card poison.
I hope this was the explanation you were looking for Earth.