This paper describes an immersive audio Network Music Performance (NMP) system designed for group singing. A prototype of this design (audio only) was deployed to ten singers across Europe, who participated in a duet vocal performance study, operating the system from their home networks. Parametric evaluation of these vocal performances was conducted in order to provide characterization of musical interactivity between performers and explore the challenges and opportunities presented for immersive audio NMP systems in practical usecase settings. Results demonstrate that it is possible to achieve performance that conforms to expectations of live interactivity and estimate the conditions under which this may be achieved. Significant effect of latency, and in one case virtual room "type," is observed across performances. Informal questionnaire responses present discussion of the potential for virtual acoustics and latency to impact the perceptual experience of networked performers.