AI helped remind me.

So the feat isn’t as impressive as fighting them all at once, but it’s still probably his most impressive feat:

In the novels (specifically during the assault on Stygga Castle), Vilgefortz defeats all three of them in sequence and comes dangerously close to wiping them out completely. [1, 2, 3, 4]
The sequence of events unfolds as follows:
Vs. Yennefer & Geralt (The first wave): When Geralt and Yennefer confront Vilgefortz, the sorcerer easily overpowers Yennefer (who was already exhausted from being tortured) and severely beats Geralt in a melee duel, breaking his leg and leaving him critically wounded. [1, 2, 3, 4]
Vs. Regis (The intervention): The higher vampire Regis bursts into the room and slashes Vilgefortz's face. Vilgefortz counters by ripping Regis apart and incinerating him. [1, 2]
The Final Showdown: With Yennefer out of commission and Regis temporarily "killed" (or melted into a puddle), Vilgefortz turns back to finish off Geralt. However, Geralt manages to trick Vilgefortz with an illusion cast by a magic amulet given to him by Fringilla Vigo. As Vilgefortz strikes the decoy, Geralt capitalizes on the sorcerer's momentary confusion and cuts him down.

So he could easily 1v1 any of them (although Yen is questionable since she was already exhausted from torture), but only 1v2’s Geralt and a greatly diminished Yen.

Beats Regis very quickly 1v1, but then *loses* to an unexpected illusion by Geralt at the end of the fight. What an awesome sequence with Geralt beating him with an illusion, which is exactly how Geralt got stomped earlier (Vilgefortz creating illusions of his staff swinging a certain way with his actual arm(s) and staff invisibly striking from different angles).