NO VULN: total damage = damage
VULN_HOLY: total damage = damage * 1.33
VULN_SILVER: total damage = damage * 1.33
VULN_SILVER + VULN_HOLY = total_damage = ((damage * 1.33 (silver)) * 1.33 (holy))

Stacking vulns only works when you have a material vulnerability + damage type vulnerability.

So long, and thanks for all the fish!