The Vampires the title is referencing aren't actually vampires, they're a sinister criminal organization/cult with more than a little bit of Jewish overtone.

Les Vampires (the people, not the movie) are sort of like a steampunk version of the kind of secret crime gang that James Bond or Batman are always fighting. The reason the girl in the bat suit is staggering around is because she's going to tell the hero all about Les Vampires after she's finished performing her ballet number but the guy you see in the tux has poisoned her. This film was a serial of about 10 or so chapters and the director had his film colored for each scene to give the audience some idea what to expect (outdoor scenes were green, murder scenes were red...). Just like today with previews fooling you into thinking a movie is something it's not, the movie posters for Les Vampires always showed the girl in her bat costume (as you can see she's only dressed up like that for 2 minutes for the entire series of films) to make you think this was a bizarre horror movie instead of a cops-n-robers movie.