There is no such thing as tacking the poison keyword onto a damageing effect and not dealing poison damage. There is no dealing fire damage with only the frost keyword. If it is fire and frost damage, then the fire and frost keywords apply and vice verse. You can't seperate the keywords and the effect.
Can you apply the poison keyword without triggering vulnerability? Sure. Any non-damaging poison effect dodges vulnerability because as you say damage triggers it.
There IS such a thing as having a poison keyworded power, however, and not dealing poison damage. Straight from D&DI:
Prismatic Spray
Wizard Attack 25
A dazzling spray of multicolored light springs from your hands, enveloping your enemies.
Daily - Arcane, Fear, Fire, Implement, Poison
Standard Action Close burst 5
Target: Each enemy in burst
Attack: Intelligence vs. Fortitude, Reflex, Will
Hit (Fortitude): If the attack hits the target’s Fortitude defense, the target takes 3d6 + Intelligence modifier poison damage and is slowed (save ends).
Hit (Reflex): If the attack hits the target’s Reflex defense, the target takes 3d6 + Intelligence modifier fire damage, and ongoing 15 fire damage (save ends).
Hit (Will): If the attack hits the target’s Will defense, the target is stunned (save ends).
Special: You make only one attack per target, but compare that attack result against all three defenses. A target might be subject to any, all, or none of the effects depending on how many of its defenses were hit. The target must make a saving throw against each ongoing effect separately.
If a power's keywords automaticly assigned that keyword to all damage, then hitting their fortitude would deal fire damage, hitting their reflex would deal poison damage.
This proves, then, that damage keywords are not automaticly the same as power keywords.
The example provided in items mentions -specifically- the keywords on damage and effects, then says that all powers contributing to that damage or effect apply.
Apply to what?
Why, the very thing the paragraph is talking about! Damage and effects!
I -really- don't understand how this isn't clear. I -really- can't get how it suddenly changes gears to something else, then changes back. It does not give the power keywords. It -might- give the -damage- keywords, which is a lot different.
If I use a torch as a weapon in a basic melee attack, it deals fire damage. However the power I use with it is not granted the fire keyword, because the -power- does not invoke fire. Astral Fire does not enhance my ability to swing torches for damage. The item modifies the -effect- but not the keywords of the -power-.
If I used Astral Fire with Prismatic Ray, however, and only hit the Fortitude defense, I'd still get the bonus damage, because Astral Fire checks for using a power with the fire keyword, not for the damage type of the power.
If I use a flaming staff for a power with the keywords cold and impliment, for example, the implement can't change the cold damage to fire damage. This is because it's not being used as a weapon, and does not deal damage. I'd get the fire critical damage tho, because that's not a property mentioning being used as a weapon. But it doesn't change the power I use to a Fire power, because the power is not channeling fire. Astral Fire won't buff it, just as the power doesn't gain At-Will, just cause one of the powers of the item has it.
This is -really- intuitive. The effect of such items is to change or add or modify the damage, so that's what gets modified. They aren't modifying the power, are they? No. They are simply changing an effect as it occurs. It's the same as if you use Shield on an attack against you. The attack against you doesn't suddenly become Arcane. If you use Elven Precision on a failed attack, that failed attack doesn't suddenly become a Racial Power.
This uses the same logic.
If I have Astral Fire, and a flaming sword, and I use a thunder weapon power with it's daily ability... the thunder weapon power does NOT get the Astral Fire bonus. The daily power of the item -does- because it -has- the Fire keyword.
And the keywords of the item don't auto-add to its damage, otherwise if I hit ya with a power through a wand of some fear spell, you'd take fear damage, and that's not a damage type.
There's too many counterexamples that make power-keyword inheritance impossible, unwieldy, or overpowered that it cannot be the case.