vanquished > {Subdued by physical or spiritual power} them > [those who were]: apparently meaning the native Britons, a statement at odds with the succeeding account (e.g. in 210) of English genealogy
6 From thence a Faery you unweeting reft,
unweeting > unsuspecting [not knowing the identity of the child; perhaps also referring to "you">[ reft > took away
7 There as you slept in tender swaddling-bands,
tender > infantile; youthly swaddling-bands > swaddling-clothes [strips of cloth in which infants were wrapped]
8 And her base Elfin brood there for you left:
Elfin brood > Faery offspring for > in place of, instead of
9 Such, men do changelings call, so changed by Faery's theft.
changed > exchanged
110.66