Vulnerable creatures take 50% more damage from Attacks.
When an enemy is Vulnerable, they will receive 50% more damage from your Attacks (75% with the Paper Phrog relic).
When the player is Vulnerable, they will receive 50% more damage from enemy Attacks (25% with the Odd Mushroom relic).
Does not affect Poisons, Orbs or Thorns. Only cards of the Attack type receive the bonus damage.
Stacks of Vulnerable only increase the duration, not the intensity. For example, 2 Vulnerable will last for 2 turns, and 5 Vulnerable will last for 5 turns, but in both cases they will take 50% more damage.