The Collectible option is for trackable items that you want to keep with you at all times. It's used mostly by cachers with Collections of geocoins that don't go out travelling from cache to cache but stay in their private collection.
If you mark your vehicle TB 'Collectible' you can then select the drop-down option 'Move to collection'. This means it will no longer show up in your 'Trackables Inventory'. Other cachers who see its tracking number will be able to log 'Discovered it' for your vehicle, or a Note log. This option will work well for your vehicle...
But...
If you want the ability to log your vehicle 'into' caches for mileage, e.g. you want to drive it to an event and log it into the event to show it was there, OR if you want to log a 'visit' for the vehicle to other caches that you may drive to, then you'll need to designate it as 'Not Collectible'. It will then show in your Trackables Inventory, all ready to be dropped off into a cache.
MrsB