What happens if I use Vampire Slam as an opportunity attack to push a creature out of the range of its attack?

Question

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?

Answer 1

Rules Compendium, pages 195-6

• An opportunity action is similar to an immediate interrupt, but it can be taken once per turn, rather than once per round.
• An immediate interrupt jumps in when its trigger occurs, taking place before the trigger finishes. If an interrupt invalidates a triggering action, that triggering action is lost.

The monster's ranged power no longer has a valid target (because of the opportunity action moving the target out of range) and is lost.

Answer 2

The power is interrupted and expended.

There are several actions in response to enemy triggers. To simplify those actions:

Opportunity Action - someone tries to hit you, but you can hit them first. If your attack kills them, their attack never happens. Example: Opportunity attacks.

PH 268: An opportunity action interrupts the action that triggered it. It completes prior to the interrupted action, and if the interrupted action becomes impossible to complete, it is lost.

Immediate Reaction - someone hits you, you instantly smack them back. If their attack kills you, your attack never happens. Example: Warden's Grasp.

Immediate Interrupt - The same as opportunity actions, but only once per turn. Example: Warden's Fury. You have only one immediate action/turn, if you use a reaction, you can't use an interrupt and vice-versa.

As you can see, in your case of an attack of opportunity with a push, because the condition to use the power (being at range max of 5 squares) cannot be fulfilled any more, the power is interrupted and spent.