Think about it...

All the pictures of the empty city streets in the early 1800's are taken during the day. Vampires would be sleeping, accounting for all the empty streets. (Can vampires be photographed? Probably not if they don't have reflections.)

Vampires would produce better art and architecture, since they'd be able to master their craft over multiple lifetimes.

It also explains why Tartarian buildings had no bathrooms. Vampires don't poop.

Orphanages? Maybe the Vampires had farms where they raised human children for food, and after the Vampires were slain, the liberated livestock needed to be rehomed.

Or maybe the Vampires weren't slain at all, and they just moved into underground cities where sunlight wouldn't be a problem, and now the entire surface world is one gigantic vampire-controlled human food farm!! Yikes!!! (That would explain why nobody remembers - it's goddamn vampire hypnosis!)