The 4th Edition Assassin class has a Level 6 Utility Power Called Vanish:
Encounter - Immediate Reaction
Personal
Keywords: Shadow, Teleportation
Trigger: You are hit by an attack.
Effect: You become invisible until the start of your next turn, and you teleport up to your speed.
My question is: does the assassin take damage from the attack that triggered Vanish? Or do they get to Vanish after a successful attack roll but before damage is dealt?
Interested to hear ruling interpretations on this.
If an attack successfully hits you, it will then deal damage. The two are linked. In addition, Immediate Reactions occur after the trigger that allows it to be used resolves. I think you are instead thinking about the "when you are attacked" trigger, often an Immediate Interrupt, which would allow you to use a power to avoid the attack altogether once the enemy declared it.
The Vanish Power is an immediate reaction. Reactions always happen after the triggering action has been affecting its target.
Would this power have been an immediate interruption instead, the trigger line would also have been something on the lines of "someone attacks you".
Trigger: You take damage from an attackto clarify this is in fact after the damage. The ability would be WAY too powerful if it allowed you to avoid damage and become invisible 1/enc. As a DM, I would probably rule against it avoiding damage even if it said it in clearer language, for balance issues. You'll find similar abilities that do actually let you avoid an attack as an Immediate Interrupt, but they let the enemy re-target a different creature with the attack, or force them to hit one of their allies (as a daily power). \$\endgroup\$