Masquerade

The Masquerade is a contract constructed between the night walking Vampire and the living. Its prime objective is to keep the knowledge of vampires hidden from the general population. With the idea of preventing wars between each side, as the vampires are strong they would wipe out their blood source if they allies themselves. Being a undesirable outcome for both the living and night walkers, the Masquerade was brought into play. 
Many scholars created research documents detailing how the previous wars and sightings of vampires were purely myth and where they needed to linger closer they just called the individuals cannibals.  Alongside this scholars also spread rumors on garlic being effective to reveal them, another tactic to further hide their identity as garlic has no effect. 
The vampires themselves formed individual clans and courts to try and prevent their lesser kind from outing themselves. These groups act as almost crime families, with some dabbling into it.  Their numbers dwindled overall, but it was a lot more stable. And those that lived tended to have a longer life span and be hired onto special jobs that ordinary mortals would suffer at.