Elemental vulnerablities double the relevant damage recieved, with the maximum increase being the size of the vulnerability. In the case of poison damage, only the initial poisoning is affected, rather than each subsequent poison tick.

There are four such abilities:

Fire vulnerability
Cold vulnerability
Shock vulnerability
Poison vulnerability

When considering capped damage, vulnerability to the relevant damage type does not increase the damage cap. For example, 1 capped armor negating shock damage can never deal more than 1 damage, even to a unit with shock vulnerability.