A creature that is vulnerable to a specified damage type usually takes a specific amount of extra damage when it takes damage of that type, or it suffers a specific effect. For example, a creature that has vulnerable 10 radiant takes 10 extra radiant damage when an attack deals radiant damage to it or when it takes ongoing radiant damage.
Against Combined Damage Types: Vulnerability to a specific damage type applies even when that damage type is combined with another. For example, if you have vulnerable 5 fire, you take 5 extra damage when you take ongoing fire and radiant damage.
Not Cumulative: Vulnerabilities to the same damage type are not cumulative. Only the highest vulnerability applies. For example, if you have vulnerable 5 psychic and then gain vulnerable 10 psychic, you have vulnerable 10 psychic, not vulnerable 15 psychic.
Published in Monster Manual 2, page(s) 219, Monster Manual, page(s) 283, Player's Handbook 3, page(s) 223.