DracoSuave
First Post
According to Page 269 of PHB damage and effects all occur at the same point in resolving the attack (Step 5 in the list).
As such, they don't all happen one by one necessarily. So the trigger is still:
"When you (push a creature or knock a creature prone), it takes damage equal to your Constitution modifier."
The bit in the brackets, added my me, is still a single trigger. If a power happens to do both simultaneously, i.e. in a single resolution (as all the damage and effects are a single event in the resolution sequence), you only get one trigger still - as A,B,A&B are still single returns of the trigger each.
If this were the case, then let's look at a typical rogue power.
Dazing Strike, level 1 Encounter power.
Hit: 1[W] + Dexmod damage, and the target is dazed until EOYNT.
If all effects were simultaneous, that would mean the target would be dazed when this power hit. That would mean if you did not have combat advantage when the attack hit, you'd have it when the damage was dealt. Which would mean you could apply sneak attack damage to this power, even if you hit someone without combat advantage.
That does not work.
Sign of Vulnerability deals radiant damage and gives the target vulnerability. Because the target would have vulnerability at the time damage is dealt, the target would take extra damage from the attack itself.
That does not work.
Also, the part you quote does not say you do damage and effects simultaneously, in fact it doesn't make sense for powers that say 'Do this, then do that, then do this'. No part of that says 'Do the effects all at the same time.'
All it says, is that resolving hits or misses is step 5. Being a part of the same step does not mean 'do it all at the same time.'
In fact, straight from PHBII:
Sequence: The order of information in a power description is a general guide to the sequence in which the power’s various effects occur.
There is a sequence (altho there are definate exceptions) and you do things in order.
Damage comes first, so you do that first. Then the push effect (which only applies once, as it is a single push effect). Then, if you pushed the target, you trigger the damage from Iron Vanguard, as you have satisfied the condition. Then you knock the target prone, if it isn't already (say from dying). Then, if you knocked the target prone, you deal damage again.
The power does not make exception, makes no mention of simultaneousness, and there's been no errata to Iron Vanguard to indicate the damage can only occur once.