Invaders of Rome (7)
I believe the answer is:
'invaders of rome' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for vandals that I've seen before include "Destroyers of the Roman Empire" , "Destructive hooligans" , "Destructive types" , "They destroy or deface property" , "they cause destruction" .)