O que fez Geralt perceber que Vereena não era uma rusalka, mas na verdade uma vampira?

Acho que resolvi, a reação de Roach ao "anel do diabo comum" não é nada de especial, é apenas o fato de Roach reagir, o que sacode a memória de Geralt e o faz perceber o que aconteceu. Ele lembra que Roach estava desconfortável quando estava perto de Vereena antes de entrar na mansão e depois ao entrar na mansão.

The mare tossed her head, neighed wildly, stamped and danced on the path, kicking up a storm of dried leaves. Geralt, wrapping his left arm around the horse's neck, swept his right hand—the fingers arranged in the Sign of Axia—over the mount's head as he whispered an incantation.

[...]

A girl was standing on the slope of the hill he had just climbed down, one arm resting on the trunk of an alder tree. Her trailing white dress contrasted with the glossy blackness of her disheveled hair, falling to her shoulders. She seemed to be smiling, but she was too far away to be sure.

[...]

Roach neighed anxiously, tossing her head. Geralt, still watching the forest, instinctively calmed her with the Sign again. Pulling the mare by the bridle, he walked slowly along the wall, wading through burdock up to the waist.

He came to a sturdy gate, with iron fittings and rusty hinges, furnished with a great brass knocker. After a moment's hesitation Geralt reached out and touched the tarnished ring. He immediately jumped back as, at that moment, the gate opened, squeaking, clattering, and raking aside clumps of grass, stones and branches. There was no one behind it—the witcher could only see a deserted courtyard, neglected and overgrown with nettles. He entered, leading Roach. The mare, still stunned by the Sign, didn't resist, but she moved stiffly and hesitantly after him.

_{The Last Wish, "A Grain of Truth" II}

No entanto, isso não poderia ter sido por causa de Nivellen como "animais como eu" e então outra coisa devia ter assustado ela. E se Vereena fosse verdadeiramente uma rusalka, Roach provavelmente não teria assustado tanto, então havia algo mais nela.

Ele também provavelmente ainda estava se perguntando sobre os cadáveres que havia encontrado na floresta antes e que haviam morrido devido a "sopra no pescoço".

The woman in the sheepskin and blue dress had no face or throat, and most of her left thigh had gone. The witcher, not leaning over, walked by her.

The man lay with his face to the ground. Geralt didn't turn the body over, seeing that the wolves and birds hadn't been idle. And there was no need to examine the corpse in detail—the shoulders and back of the woollen doublet were covered with thick black rivulets of dried blood. It was clear the man had died from a blow to the neck, and the wolves had only found the body afterward.

_{The Last Wish, "A Grain of Truth" I}

É só quando Geralt está saindo que Nivellen diz a ele que os dois estavam lá na mansão anteriormente e que ele realmente não teve tempo, ou quer questioná-los sobre isso.

“I’m not as stupid as you think. You came here following the tracks of one of the merchants who'd been here lately. Has something happened to one of them?”

“Yes.”

“The last was here three days ago. With his daughter, not one of the prettiest, by the way. I commanded the house to close all its doors and shutters and give no sign of life. They wandered around the courtyard and left. The girl picked a rose from my aunt's rosebush and pinned it to her dress. Look for them somewhere else. But be careful; this is a horrible area. I told you that the forest isn't the safest of places at night. Ugly things are heard and seen.”

_{The Last Wish, "A Grain of Truth" II}

E assim, lembrando-se de Roach estar assustado e dos cadáveres que morreram com ferimentos no pescoço, ele juntou dois e dois para perceber que Vereena era uma vampira. o "anel do diabo comum" pode não significar nada específico, mas os vampiros costumam ser acoplados ao diabo e isso pode ter ajudado Geralt a fazer a conexão.