Integrating biofeedback technology, such as real-time eye-tracking, has revolutionized the landscape of virtual reality (VR) rehabilitation games, offering new opportunities for personalized therapy. Motivated to increase patient focus during rehabilitation, the Focus-Driven Augmented Feedback (FDAF) system was developed to enhance focus and maintain engagement during upper limb VR rehabilitation. This novel approach dynamically adjusts augmented visual feedback based on a patient's gaze, creating a personalised rehabilitation experience tailored to individual needs. This research aims to develop and comprehensively evaluate the FDAF system to enhance patient focus and maintain engagement in VR rehabilitation environments. The methodology involved three experimental studies, which tested varying levels of augmented feedback with 71 healthy participants and 17 patients requiring upper limb rehabilitation. The results demonstrated that a 30% augmented level was optimal for healthy participants, while a 20% was most effective for patients, ensuring sustained engagement without inducing discomfort. The research's findings highlight the potential of eye-tracking technology to dynamically customise feedback in VR rehabilitation, leading to more effective therapy and improved patient outcomes. This research contributes significant advancements in developing personalised VR rehabilitation techniques, offering valuable insights for future therapeutic applications.