This concept intrigues me.
The ‘Bloodworld’ being, of course, ours. The vampires are only somewhat from this planet: the rest of them comes from the dark, cold, and dry depths of space. Bloodworld Vampires are a terrifying combination of infectious agent, spy, and harvester. But they’re not here to invade, any more than the farmer invades his orchard.
When it comes right down to it, the depths of interstellar space is the perfect place for a vampire. The stars — which are, of course, suns — are comfortably far away. There’s no running water, no silver in refined form, no blessed woods, and no fire. And the cold; the wonderful, preserving cold. A vampire can drift for centuries on a single body’s worth of blood, and cultivate strange and disturbing gore-rich bloodgrowths on the comets that drift through the outer darkness of the stellar systems. This generally suits them well enough.
But the strains of bloodgrowths can grow weak with over-cultivation, so fresh sources of nourishment must be periodically found and assessed. To do this, the vampires of the comet clouds send down special parasites into the solar gravity wells. Their task is to find suitable members of the local sapient specie to infect into ‘classic’ vampires; once one is found, the new Bloodworld Vampire will drink widely from a variety of vessels, the better to get a taste of new and exciting strains of life.
Once they’ve gathered enough, they… get it back up the gravity well somehow. Vampires in this setting, despite their ostensible science fiction aspect, are not entirely mundane creatures. They are instead supernatural creatures who take advantage of the laws of physics when it’s useful to do so. The point is, Bloodworld Vampires have a plan, and an internally consistent reason not to turn the entire world into nosferatu.
This makes them interesting as a gaming concept.