I can't seem to find any answer on whether or not vulnerabilities apply to each damage roll, each attack roll, or each attack power. The main issue at hand is when an attack power contains multiple individual attacks. If each attack hits the target, do you add (or minus) the resistances to the TOTAL damage that is dealt by the power, or the damage to each individual attack?
Take for instance the Ranger At-will power "Twin strike", that contains two attack dice rolls. Each attack roll grants a damage roll.
Attack: Strength vs. AC (melee; main weapon and off-hand weapon) or Dexterity vs. AC (ranged), two attacks. Hit: 1[W] damage per attack.
If a ranger manages to hit a target with both rolls and that target is vulnerable 10, does the target take 1[W] + 10 + 1 [W] + 10 damage (2[W]+20), or does the target only take 1[W]+1[W]+10 damage (2[W]+10).
The same issue will probably apply to resistances as well.