It's iZombie with vampires.
IT'S IZOMBIE WITH VAMPIRES.
(Sorry, do you need a link to previous yelling?) It's not vampires and a drug that mimics vampirism, but drug-induced vampirism in which some people got an early taste.
And then there's a shoddy attempt at Westworld sleight of hand, except Westworld's temporal trickery ties into and reinforces themes, while Let the Right One In just fumbles the obvious vampire infection-as-addiction/addiction-as-sickness routine because to actually lean into it would require an unbowdlerized approach to the vampire lead, where they let her be in some way messed up by her illness/addiction and not just vaguely grumpy about having to move all the time.
They can't even bend enough to let the 'sick child vampire' be sick enough to carry through on the metaphor because they're too committed to absolutely family-proofing the story.
Their temporal trick doesn't do anything but try to be clever on top of a twist that's trying to be clever, apparently all a very obvious attempt to distract the audience from the question of why they didn't just adapt the actual story.