A vandal is someone who breaks or damages things on purpose, for no good reason — smashing bus-stop glass, keying a car, spraying paint on a wall, tearing up seats in a train. The key idea is ‘on purpose and pointless’: a person who breaks a window by accident is not a vandal, and a thief who breaks a window to steal something is a thief. It is a countable noun, usually plural in news writing (‘vandals attacked the park last night’), often left vague because nobody knows who did it. The word is always negative. Where the line falls is argued about — some people call a graffiti artist a vandal, others call the same person an artist, and that argument is really about whether the damage means anything. The word comes from a real people, the Vandals, who attacked Rome in the year 455; later writers blamed them for wrecking beautiful things, which historians now say was probably unfair to them.