Hey Ryen, can you tell me where I can obtain the Jewelry recipe from Sandra... i.e., what needs to be done to gain it and is this what is needed to unlock Kuro for the vanguard? Thank you
What jewelry recipe? If you mean the jade recipe, you get it by advancing the events of Sandra. There is a step-by-step guide for her events in the walkthrough, if you need help with that.
In order to recruit Kurohime as a squad leader, you need to tell Theremis to prepare a war against Dorgania, and you need to advance the events of Grey to have the meeting with Lady Akira. When all necessary conditions are met, and Kurohime is in your service, a spy mission will become available at Luolambo on the country overview map. This spy mission unlocks her as a squad leader.