Due to the features of immersive learning and without the restrictions of time and place, virtual reality (VR) is commonly used in learning areas. For investigating functional design in the VR learning environment, previous literature proposed a series of theoretical models and strategies without considering the key factors for their development. Therefore, this study explored the key factors based on the dynamic ADDIE model for the design of VR instructional systems. To augment the assessment integrity, this study adopted the fuzzy Delphi method (FDM). Open and closed questions were also integrated into the online FDM-based questionnaire to assess the opinions of expert groups, with 9 professors and 9 school teachers. The results showed that the final selection encompassed 5 criteria, 2 sub-criteria, and 23 key factors. In the overall priority order, the "interactive feedback design" was ranked as the most influential factor, while the "learner opinion survey" was the least. Most importantly, since the experts considered "formative evaluation" to be the highest significant sub-criterion, the reliability of the dynamic ADDIE model was confirmed. In addition, the divergent opinion from expert groups could provide references for future developers with different backgrounds.