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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.

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.