You can overcome an innate vulnerability through sheer willpower.
Prerequisites: Vulnerability to energy, Iron Will, Suppress Weakness.
Benefit: You can completely suppress your vulnerability to a type of energy. When subjected to an attack based on that type of energy, you take no extra damage.
Normal: A creature vulnerable to a type of energy takes half again as much (+50%) damage as normal from that energy type, regardless of whether a saving throw is allowed, or if the save is a success or failure.
Source: Draconomicon, page 72.