Let's start with a basic idea.
The Vampire RP
The setting would be some kind of city, probably a large expanse of countryside as well with a few small towns. There is an organization of vampires that regulates vampire stuff and deals with problems. There are hunters that try to kill vampires. One could assume that there are also evil vampires that hope to destroy the humans, impure vampires or something like that, or maybe they're just crazy and want to rule everything.
Let's see, there would be some vampire infighting between the organization and the vampires against it. They would all be fighting the hunters, which can also have some sort of organization if you want. The hunters and the vampires both would want to stop an evil vampire... but probably would not work together. Pretty basic, may need some fleshing out.
What else... garlic does nothing to vampires. Holy symbols are laughable to an older vampire, but may cause some discomfort to a newer one. Beheading will kill a vampire, stake to the heart, maybe some kind of fancy silver nitrate deal in high enough amounts will also kill one. New vampires are made via a simple process: A vampire bites a person, the vampire then has the person they bit ingest some of their vampire blood. That's about it.
The plan I had was to keep most of the vampire stuff simple, no crazy over elaborate stuff. It's supposed to be simple and fun, the lore is all unique to this RP so almost no outside knowledge is needed save for what a vampire is. Think Buffy the Vampire Slayer, it's simple like that. Vampires drink blood, they don't like the sun, they are hard to kill, and they are probably strong.
Now you throw your thoughts into it and discuss.
REMEMBER, simple is better. This RP is not intended to emulate a "real" vampire experience, it is supposed to be fun and thought provoking. Fun first, facts second.