Higher vampires bear an uncanny resemblance to humans. Yet the similarities are only skin deep. We mere mortals have a dastardly time trying to understand their emotional makeup and morals. So Geralt could never have foreseen what happened - that Dettlaff would try to take Syanna's life.
Luckily, Syanna had her protective amulet. Crafted long before by Artorius Vigo, it did exactly what it was created to do. At the crucial moment, the duchess' sister disappeared, transported to a spot both safe and secure. Dettlaff flew into a rage. He lunged at Regis and Geralt, intent on tearing them to shreds. Yet the duel ended with his defeat and, at the last, his demise.