The power in question, Vampire Slam, is:
Vampire Slam (Vampire Attack 1)
At-Will ✦ Implement, Shadow
Standard Action Melee 1
Target: One creature
Attack: Dexterity vs. Reflex
Hit: 1d10 + Dexterity modifier damage, and you can push the target 1 square.
Special: You can use this power as a melee basic attack.
(Heroes of Shadow page 53)
Now, this power can be used as a melee basic attack and can thus be used as an opportunity attack.
Suppose a monster wants to use, say, a Ranged 5 power when exactly 5 squares away from one of the Vampire's allies when the Vampire is adjacent to the monster. The ranged power provokes an opportunity attack from the Vampire, who uses Vampire Slam to push the monster so that it is now 6 squares away from the Vampire's ally, and now the Vampire’s ally is out of range of the monster’s attack.
What happens now?
Does the power function normally? Does the power fail, and if so, if the power was an encounter power, is it expended?, If the power fails, is the action that used the power lost?