If I hit a vulnerable target with an effect that hits it twice, such as Static Charge or Chromatic Bolt, does it count the damage for vulnerability twice?
The simple answer is yes
I don't have my rule books to hand, but I remember this coming up in our 4e game. What we found out is that a vulnerability is applied to each source of damage. If a power grants you two attack rolls, i.e "Twin-Strike" (Ranger at-will) then they are counted as separate sources.
I think we couldn't find a direct ruling for it, but take for example "Twin-Strike" that lets you target two different creatures 'A' & 'B'
Your character is equipped with +n Flaming Sword x2 (required for Twin-Strike in melee) You attack creature 'A' which has vulnerability fire 'X' with your main-hand and creature 'B' which also has vulnerability fire 'X' with your off hand and hit both creatures. You roll for damage and apply the fire damage from your weapons, causing them to take 'X' additional damage.
This is rules as written. So logically why would using the same power on creature 'A' alone and hitting it twice only cause it to take it's vulnerability once?
Note this also applies to resistances!
Though this is coming from the same source each round, the vulnerability will still apply each time it is taken.
Hit: 1d6 + Constitution modifier radiant damage. If the target moves nearer to you on its next turn, it takes an extra 1d6 + Constitution modifier damage.
This extra damage will not trigger the Vulnerability as it is un-typed damage. However if it stated "it takes an extra 1d6 + Constitution radiant damage" it would trigger the vulnerability.
Rain of Blows:
Hit: 1[W] + Strength modifier damage. Weapon:
If you’re wielding a light blade, a spear, or a flail and have Dexterity 15 or higher, make a secondary attack.
Secondary Target: The same or a different target
Secondary Attack: Strength vs. AC Hit: 1[W] + Strength modifier damage.
Similar to "Twin-Strike" in the fact that it is two separate attacks. But what is different here is that in Twin-Strike the attacks happen simultaneously, these and sequential.
Hit: 1d8 lightning damage, and one enemy adjacent to the target takes lightning damage equal to your Charisma modifier.
If the adjacent enemy had lightning vulnerability it would be applied despite the fact that no attack roll was made against them directly. They still took damage from a source, you can just assume here it is an auto-hit roll.
Hit: 1d8 + Charisma modifier psychic damage, and one creature within 5 squares of the target takes psychic damage equal to your Constitution modifier.
This is the same as Static Charge as above with one interesting caveat. The second creature cannot be the original target causing it to trigger the vulnerability twice.
Rather than just say "because" I will attemp to explain why.
In Static Charge it states "one enemy adjacent to the target" This can also be written as "one enemy within one square of the target" So for the same reasons that you do not count an enemy adjacent to itself, you do not count it within "X" squares of itself.
As mentioned in the comments WotC support have apparently stated that a creature is within "X" squares of itself for the purpose of the second target. If this is the case it will trigger vulnerability twice rules as written.
This damage is not additional damage and instead works more like an auto-hitting attack roll counting as a new source.