Virtual reality applications turn abstract concepts into experienceable phenomena and present exciting opportunities to transform science education and public outreach practices. While research has started to look into the affordances of virtual reality (VR) in the formal science education context, the potential of these technologies to enhance public engagement with science is largely unexplored. To improve the way that VR may be used in informal learning and public outreach contexts, the purpose of our study was to undertake evidence-based investigations that shed light onto the relationship between VR and public engagement. Aiming to identify and develop the benefits of VR technologies, we propose a conceptual framework for engagement with VR at a science festival that comprises four aspects of participant activity: "immersion," "facilitation," "collaboration," and "visualisation." This framework guided the research design of our exploratory case study of one VR tour at a science festival. Data included visitor surveys, video recordings, VR screen captures, and focus group interviews with outreach and science professionals. Our findings reveal important ways that VR supports visitor engagement at a science festival. More generally, these findings and our framework contribute to the ongoing efforts of engaging the public with science in more diverse informal learning contexts.