The key issue seems to not be the fiction but that the elements creating your “vampire” are separate. Your Masai with xeroderma pigmentosum has vampiric properties because of distinct separate events. If there were say a single virus that made people both have a similar light aversion and made them desire blood, I don’t think most people would have a problem calling them vampires.
Indeed, I would not object to being called a vampire if I had porphyria. (I was going to write “call someone a vampire if they have”, but I realize they might conceivably find it offensive.)
The key issue seems to not be the fiction but that the elements creating your “vampire” are separate. Your Masai with xeroderma pigmentosum has vampiric properties because of distinct separate events. If there were say a single virus that made people both have a similar light aversion and made them desire blood, I don’t think most people would have a problem calling them vampires.
Indeed, I would not object to being called a vampire if I had porphyria. (I was going to write “call someone a vampire if they have”, but I realize they might conceivably find it offensive.)