IANAL or a resident of California, but I think you're right. According to California Penal Code § 594 PC, vandalism is when a person either "Defaces with graffiti or other inscribed material", "Damages", or "Destroys" property, which doesn't describe this act. I would think some sort of "mischief" is the most they could reasonably charge someone with. Maybe "public endangerment" but if you did this to a driver, they would just remove the object, so I think there's an argument of who is causing that public endangerment.