Could Sans even become a vampire in the first place? The Warden didn't have any blood, but perhaps the magic of vampirism could move past that little problem. Although, come to think of it, the Psionic wasn't a vampire either and he certainly must have different blood than the more human members of the Carnival.

Sorry Elsa, Strange just stars into space for a moment as he mulls over vampirism before realizing oh wait, she asked a question, he probably should actually answer that.

"Oh! Terribly sorry, you asked a question. To the best of my knowledge, Joker was the only supervisor turned. Of course, I could be wrong, I don't encounter a few of the supervisors on a regular basis."