A: people commit acts of vandalism to mark their territory, to lash out at business or places, to spread fear or a message to people who will see it.
B: there’s not always a way to change vandalism even if you make laws. People break and push the rule and see how far they can stretch the law.
C: I would report both situations because both acts being done are wrong. It does not matter how good of friends you are with the people or how mad they could get. It might be easier to report the random person but you need to report both situations. If you do then you have integrity which is doing the right thing when no one else is watching and you are more of a trust worthy person.
Hope this helps!