(To be replaced after New Year ;-;) There were a couple of things that Lukas surmised from how the Vampires operated:
First of all, they were not Undead or any species related to it. Sure, they needed blood to live, but they still rested and slept like ordinary humans.
Second of all, they held no Immortality. Or rather, they no longer held the capacity to live forever. A lot of Vampire Overlords were killed by humans even, based on how they referred to the war in the past.
Third of all, all things considered–there was still a chance that he was wrong. That maybe his accumulation of knowledge would contain discrepancies and inaccuracies.
Lukas held the dagger that contained Drusilla, ancestor of the Sanguise House. She had spoken about Dragons, Demons and the court of the Fae and even had a powerful aura ability while she was trapped within the weapon.