A creature with vulnerabilities takes half again as much damage (+50%) from a specific damage type (usually a type of energy damage), regardless of whether a saving throw is allowed or if the save is a success or failure.
Creatures with a vulnerability that is not a damage type instead take a –4 penalty on saves against spells and effects that cause or use the listed vulnerability (such as for creatures with vulnerability to the light descriptor). Some creatures might suffer additional effects, as noted in their descriptions.