As I understand it, vulnerabilities are stackable, but the curve is not exponential.

For example, if a race was vulnerable to a metal and an element, using something that had one or the other would hurt them more than using a weapon against them that had none of their vulnerabilities. But, if you had a weapon that had both the metal and the element, it would hurt them more than one single vulnerabilty weapon, but not twice as much. From a number perspective, it would be like normal weapon = 1.0 damage, one vuln weapon = 1.5 damage, two vuln weapon = 1.75 damage.

There are weapons that exploit three vulns, and it would be maybe something like three vuln = 1.95. Its sort of a law of diminishing returns situation. You're still limited by the avg. damage output of the weapon, but sometimes if you can exploit multiple vulnerabilities, its slightly more favorable than wielding a higher avg. weapon with no vuln exploits. Part of game balance and all.