Suppose I cast a Vampiric Link on an opponents creature. Does the opponent gain life when this creature does damage or do I gain that life?
Vampiric Link says:
Whenever enchanted creature deals damage, you gain that much life.
"You" on a card refers to the controller of the card - and regardless of what Vampiric Link is enchanting, you (the person who cast Vampiric Link) will always* control it. So you'll always be the one gaining life, not your opponent.
*barring any control changing effects that occur