I am going to guess what you are doing. It sounds like you want to change your retainer's job and you can't do that because your retainer already has a job?
The item "modern vocation" is a job crystal for your retainer. To use an example, if your retainer is a black mage and you want want to change them to a scholar, "modern vocation" will do nothing. If you want to change your retainer's job, you have to re-set their class and start over as level 1. The information from the retainer's previous job is not saved.