Vista ships, like the Zuiderdam do not have laundry rooms.
You are not allowed to bring irons on ships, so don't even try as it will be confiscated, when they scan your luggage. Some people bring steamers and find this works.
You can send all laundry and dry cleaning out to the ship's laundry. Just speak to your cabin steward and he will take care of it for you. They don't charge very much for this service.
The reason they don't want paxs to have garment bags with the hooks poking out is that the hooks could get caught on other baggage and cause a lot of damage. Of course if you are driving and will be carrying the garment bag on board with you, that that's a different matter. Even the airlines will not accept garment bags with the hooks sticking out.