inthegreylight
Explorer
Weakness to holy symbols seems well represented by the cleric class' Turn Undead feature (and the devotion paladin's Turn the Unholy feature). Maybe vampire spawn could have disadvantage on saving throws against features that turn undead. I like the idea of common folk believing a holy symbol will protect them from vampires, not realizing that it's the holy symbol used in conjunction with Turn Undead that actually lets clerics drive away vampires.