I have never heard of vacation pay on a cruise ship. Generally everyone is under a service contract that lasts for several months. When the contract ends you are no longer employed. You may have a new contract starting in 8 weeks (it varies) but that is a separate employment agreement.
It is only called "vacation" but it is really just the end of your current contract.
There may be exceptions, especially for NCL-America, but I have not heard anything specific.