In this AU, the Travelers are both "holy" beings and the Heavenly Principles' original inspiration for Angels. When Aether absorbed the Abyss in this AU, the Abyss and his body react in a similar way to Ley lines with the Abyss — rapid evolution (in-game examples: weirdly convenient local mechanics, dark mud as explained by Nahida).
Vampirism is a metaphor for intense love or passion, but this has been overtaken by ambition and greed.
At its root, it is a mutated version of the Abyss combined with Aether's undying devotion, that was then used and absorbed by the 5 Sinners of Khaenri'ah as per canon, making them the known "progenitors" of Vampires. They're more or less Aether's first "victims," alongside Dainsleif, who Aether can't bare to ever look at again, like Lumine.
(Dainsleif and Aether both yearn for each other pathetically)
Basically vampires r pathetic yearners and biting is their equivelant of intimacy, but when you only yearn for more power, they become metaphorical rapists that bite everyone w/out consent. Do note these bites are not inherently sexual? They can be familial or, like the Jinni in Genshin, can be a violent mind-shattering love-hate, bc love is something that is hard tk define and label. Aether's "beloveds" are Khaenri'ah and Lumine, both of which are entirely familial but not any less all-consuming.
The Abyss Order is its own organization here. He may be a "good" vampire, but he is a huge enabler here because he caused them thag suffering.