Is it possible to do a vinyl fence with differing colors on each side?
Vinyl takes paint. Scuff-sand it with a green 3M Scotchbrite pad to give it some roughness for the paint to engage.
Then prime with alkyd enamel.  Roller and brush it carefully so you don’t throw or drip any on their side. I would use white primer such as Rustoleum 7780 so if you have boo-boos with the primer they won’t be obvious. Then a latex/emulsion topcoat is fine.  Latex is much easier to clean up!