Elemental Blast (Sp): At 9th level, you can unleash a blast of elemental power once per day. This 20-foot-radius burst does 1d6 points of damage of your energy type per sorcerer level. Those caught in the area of your blast receive a Reflex save for half damage. Creatures that fail their saves gain vulnerability to your energy type until the end of your next turn. The DC of this save is equal to 10 + 1/2 your sorcerer level + your Charisma modifier. At 9th level, you can use this ability once per day. At 17th level, you can use this ability twice per day. At 20th level, you can use this ability three times per day. This power has a range of 60 feet.
How would this vulnerability work with creature immune or resistant to said energy type?
Unless I'm missing something obvious, immune creatures would be immune to the entire affect, while resistant creatures would apply their resistance against the damage.