A player can choose one vehicle at a time to be their "priority vehicle". As long as the player is online or doesn't change to another priority vehicle this vehicle will not despawn.
This can be placed as a button in the services menu (like "set current vehicle as priority vehicle"). Job vehicles can not be prioritized.
Edit: Some job vehicles should be allowed on second thoughts. For example, there are no not-job-related planes so they should at least be allowed.