JKR’s “authorial voice” is suppressed. She mostly writes in the casual dialect of her characters, to give a sense of how they experience the events of the story.
The very long noun phrase about the vampire conjoins two propositions about that creature:
he’d met the vampire in Romania
he was afraid [that] the vampire would be coming back to get him one of these days
To ‘package’ this information efficiently, JKR makes each a relative clause modifying a vampire. These might in a more formal style be headed by which or that, but JKR prefers a more conversational ‘null relativizer’ (represented here by Ø). This replaces the vampire in each proposition, and moves to the head of the clause (a _ marks the place from which it was moved):
Ø he’d met _ in Romania
Ø he was afraid _ would be coming back to get him one of these days
The conjunctive construction permits the second ‘he’ to be ellipted, since it occupies the same position and structural role in both clauses:
’d met _ in Romania
was afraid _ would be coming back to get him one of these days
Technically, vampire is not the subject of would be coming back; that subject is the invisible null relativizer Ø.