The vampire book is concerned with far more recent vampire history and is largely foccussed on the vampire count wars with the empire. Basically Nagash conquered Nehekara through assasination and co-ersion, then he created the elixer of life and made the vampires. After he was toppled from power he fled north and built Naggashizzar and then worked the spell that destroyed Nehekara leaving them all dead. However due to the charms over the dead tomb kings they returned to unlife free of Nagash's power. They united and destroyed Nagash's forces again and the Lahmian court of vampires too causing them to flee north, hence vampires in the empire.