Wispdrinker Vampire's activated ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't get deathtouch or lifelink.

Wispdrinker Vampire's triggered ability checks the power of a creature only at the moment it enters the battlefield. If it enters with counters, those counters are included. If that creature's power is 2 or less when it enters the battlefield but becomes greater than 2 after the ability triggers, each opponent will still lose 1 life and you'll still gain 1 life.