Danny makes a Dracula joke to Vlad, and Vlad reacts so explosively that Danny decides to spread vampire-related rumors about around Amity Park that Plasmius is a vampire.
He does it as Phantom. He thinks he’s subtle and sneaky about it: informing the public they can ward off the Wisconsin Ghost with garlic and crosses, really projects when he makes vampire puns when they fight, etc.
No one in Amity believes him. They go along with it anyway because they’re at a point where pretty much everyone likes Phantom, or at the very least recognize they owe him one. Also, Plasmius is terrible.
They shout “oh no, a vampire!” when he’s spotted, stage whisper “How is he out and about before sundown?” if he’s nearby, and the local paper even runs an article with ‘research’ on the history of “Ghostferatu” and a complete guide on protecting yourself from vampires at the bottom.
At first they don’t tell Phantom that they don’t really believe him. After a while someone lets it slip that they know, and Danny realizes he has an inside joke with the whole town. It’s twice as sweet since it’s at Vlad’s expense.