Come to think of it, is Cazador being a vampire lord supposed to even be a secret in the city? Or is it one of those open secrets like "capitalism bad" or "royalty are all parasites"? I seem to recall some banter from Gale that implied he knew before Astarion even mentioned his name.
Hardly open to the entire city, but clearly known to Baldur's higher ranking officials, and there were rumors among the common people as well. Cazador's human slaves, people who came to beg him for the gift of immortality, where would they come from if it were a real secret? Cazador's receptions were attended by high-ranking guests. The authorities always know about such things, and there was likely some sort of unspoken agreement.
Knew, of course, not everyone did. I had the Baldurian background and Tav had the line, “That big gothic mansion was full of vampires this whole time?”