Call the cops!!!! Around here vandals just need a little time in jail to straighten them out. If there is enough damage to constitute a felony criminal damage to property and your wife and friends are hardcore, then a little prison time may teach them a lesson. Otherwise sand, wash, and brush like you would have normally done with prep. Wash and brush with bleach or something for the mildew and any mold and then use an additive in the paint. Caulk everything also. Talk to you paint store about whether you need a primer or not. It can't hurt and I would suggest it but it depends upon your prior surface.
In the old days, we made vandals clean up and fix their messes. So after they have made your ice tea and prepared your hammock to supervise from, then put them to work straightening out their mess. Watch out about painting in humid and freezing weather so unless you are in florida counting votes, you may be stuck waiting for good weather.