As an addendum - it only seemed to be that way for the example pictures - not sure why they wouldn't delete.
I created another vehicle and add some photos - and then deleted the vehicle and it worked fine.
HOWEVER - there is one picture that still remains that ends in _gallery.jpg that doesn't get deleted. I was able to fix this issue by editing the administrator/components/com_vehiclemanager/admin.vehiclemanager.php in three places.
Where there is the unlink(.... "_mini"
... I copied that line and changed the _mini to _gallery.