As long as it's your turn, creatures and planeswalkers you control have lifelink.
[+2]: Sorin, Vengeful Bloodlord deals 1 damage to target player or planeswalker.
[−X]: Return target creature card with mana value X from your graveyard to the battlefield. That creature is a Vampire in addition to its other types.
If Sorin leaves the battlefield after you’ve activated his first loyalty ability during your turn but before it resolves, you’ll still gain 1 life as that ability deals damage.
If a card in a graveyard has {X} in its mana cost, X is considered to be 0.
Multiple instances of lifelink on the same creature or planeswalker are redundant.