Vita3K supports NoNpDrm and FAGDec dumps. You can manually drag and drop decrypted DLC folders directly into your emulator's pref_path/ux0/addcont/[TitleID] directory.

Navigate to Vita3K/config/config.yml . Open it with a text editor and search for addcont . You should see entries listing your installed DLC by Title ID. If the entries are not there, the emulator didn't register the license.