Owned vehicles are now persistent and no longer need to be stored. Vehicles that are left in the world are automatically saved and will be there for you when you come back to them with few exceptions:
A vehicle that is illegally parked may be impounded by the Los Angeles Police Department Parking Enforcement Bureau. This included vehicles parked in red curb no parking/stopping zones, vehicles left unattended on a roadway, parking on someone's private property, etc. Vehicles impounded will be brought to the Los Angeles Police Impound.
While offline, a personally owned vehicle is safe from damage and theft. However, anyone with a key will be able to use your vehicle still, and it will no longer be protected until that person logs off, so ensure you trust those that have a key to your vehicle.
If you do not wish to leave your vehicle out, you can still use the garage system to store your vehicles.