Graduate Teaching Assistants (GTAs), or student instructors, are the crucial force in college for undergraduates' learning in Science, Technology, Engineering and Maths (STEM) disciplines. However, professional development of student instructors is often neglected. Providing adequate and appropriate teacher training for student instructors is a critical challenge. When the technology is available, open-source non-immersive virtual reality (VR) can be a cost-efficient and accessible platform for teacher training. Empirical research of designing and implementing VR for the training on teaching knowledge and skills development is inconclusive and thus warranted. In this ex post facto study, we investigated VR-based teacher training with 33 STEM student instructors to explore the effects on the participants' virtual teaching practices of two design factors: (1) the simulated teaching scenario and (2) the duration of training program implementation. We analysed 7604 event logs from the recordings of their virtual teaching sessions. The results of ordinal logistic regression analyses showed two factors contributed to higher odds of appropriate teaching actions. The first is the simulated scenarios that induced a more dynamic balance of domain-specific and pedagogical knowledge for decision making in teaching; the second is the teacher training program with a longer duration.