Welcoming Vampire's ability checks the power of the other creatures only as they enter the battlefield. If one of those creatures has power 2 or less, the ability will trigger. Once the ability triggers, raising that creature's power above 2 won't affect that ability. Similarly, reducing a creature's power to 2 or less after it enters the battlefield won't cause the ability to trigger.

If creatures enter the battlefield with +1/+1 counters or a continuous effect such as that of Wedding Festivity will apply to the creatures on the battlefield, those effects apply when checking to see if Welcoming Vampire's ability will trigger.