Intriguing. I had not previously run into references of people blaming plague on vampires, but it certainly makes sense. I wonder if there was any regional variation in the prevalence of the belief.
What fascinates me is that this seems strange for Italy. There is some folklore regarding vampires as spreaders of plague, but those are more typical to Eastern Europe. IIRC, there wasn't much vampire lore that came from Western Europe (there was a 'vampire scare' in New England during the 18th and 19th Centuries but that was a rather isolated incident) which adds on to how intriguing it is that this story came from Italy.