Southwest Vacations is outsourced to a tour company and is not part of Southwest Airlines. This is typical of airline vacation packages, they are not the only one.
Airline credits are only accepted for straight air purchases, not for the air portion of the vacation package since it's a different fare structure in the packages and a different entity that is offering those packages and processing your payments.
Now you CAN use your credits to purchase separate airline tickets and book the Southwest Vacations package as a "land only" package, but the total pricing will most likely be different and some vacation package promotions may require the air be purchased as part of the package.