Being vulnerable to a damage type means a creature takes extra damage from that damage type. Vulnerability appears in a stat block or power as “Vulnerable x,” where x is the amount of the extra damage. For instance, if a creature has vulnerable 5 fire, it takes 5 extra fire damage whenever it takes that type of damage.
Against Combined Damage Types
Vulnerability to a specific damage type applies even when that damage type is combined with another. For instance, if a creature has vulnerable 5 fire, the creature takes 5 extra fire damage when it takes ongoing fire and radiant damage.
Vulnerabilities to the same damage type are not cumulative. Only the highest vulnerability applies.
Example: If a creature has vulnerable 5 psychic and then gains vulnerable 10 psychic, it has vulnerable 10 psychic, not vulnerable 15 psychic. Similarly, if a creature has vulnerable 5 psychic and then gains vulnerable 2 to all damage, the creature still has vulnerable 5 psychic, not vulnerable 7 psychic.
Combined with Resistance
If a creature has vulnerability and resistance to the same type of damage, they both apply. Subtract the smaller value from the larger one and apply the result. For instance, a creature that has vulnerable 5 fire and resist 10 fire is treated as if it has resist 5 fire.
Published in Rules Compendium, page(s) 225.