LORE (Gist)
Lira became a vampier when she gets mixed up in a rich gathering of vampires.
And once her parents died she had to protect herself from all the others , including other vampires.
SPECULATIONS
So clearly she was not born vampier, she got converted into one, unlike the TRUE Vampire like the dark eternals who chose to abandon the dark codex. Yes, I'm talking about the vampier monster we know.
The true vampires wanted to increase their numbers by bringing in powerful humans on their side which backfired as Lira turned out to have unique abilities which were superior compared to the true vampires, being able to move from shadow to shadow and being able to attack from a distance...The Dark Eternals felt threatened by her existence and tried hunting her down, but the difference in power was too much, and decided to go into hiding instead and now she comes out of hiding as the true vampires cower in fear!