Dracula won, and you live in the world He built. Europe’s upper classes converted to vampirism en masse, and continued to rule their ancestral holdings with the unhallowed blessing of the Red Emperor. Consider the following to be the most important setting principles rather than an exhaustive lore primer.
This isn’t a secret; everyone knows. You can’t miss them. They’re stronger than humans, faster than humans, more stylish than humans, and very hard to kill.
They have the standard vampire vulnerabilities: sunlight, garlic, silver, certain religious icons. They don’t have reflections, and they can’t cross running water, unless they have grave earth under their feet. They can’t enter buildings uninvited (this one is important). They need to drink blood to sustain their immortality, but most of them still enjoy eating and drinking regular food as well.
Vampires are petty egomaniacs who hold grudges forever. All the major vampires hate each other, but since they’re disinvited from each other’s houses they have to get creative with how they pursue these feuds. Often this is where the players come in.
The Great Vampire Houses are served by legions of mortal suck-ups and sycophants, desperate to gain the approval of the Red Lords and Ladies, and maybe - just maybe - be granted the gift of immortality. The Great Houses have been closed to new members for quite some time, but this doesn’t stop anyone hoping.
Business takes place at night, all buildings have shutters that totally exclude sunlight, and the cultivation of garlic is forbidden on pain of death. Humans are prohibited from carrying weapons that could pierce a vampire’s heart.
Vampires hate change, so it has been the 1880s for over a century, mixed with a dash of black magic and steam-powered super science. Think zeppelins, tesla coils, very large hats, mutton chop sideburns, corsets, steam trains, steam ships, cobbled streets, gas lamps, stage coaches, Dickensian orphans, and those adverts where they spend 1,000 words describing how a bar of soap works. Photography exists, but hasn’t attained the same prestige it did in our world, since vampires don’t show up in photos.
The Red Emperor made his last public appearance over fifty years ago. Rumours abound over what happened to Him, but nobody’s sure. His imperial flag still flies above his citadel in London, so presumably He is in residence. In His absence, the Great Houses jostle frantically for control of the Undying Empire.