Is a highway billboard vandalism when the owner of that billboard is allowing the ad?

Or are you referencing computers with ads in them (websites, etc.)? By that definition, anything disproving a conspiracy theorist is vandalism to them. That’s not how that works.

Vandalism usually means permanent damage. You can call the police on your neighbor for playing loud music at 3 am, but I don't think it would be correct to accuse them of vandalizing your property.