TESOL (Teaching English to Speakers of Other Languages) training programmes in Vietnam often emphasise the significant alignment of academic knowledge with practical classroom methods. However, an intricate yet underexplored topic is understanding the dynamic engagement of student teachers in those programmes to investigate the complexities of such alignment. Therefore, this research examines factors influencing student teachers' in-class engagement following a 120-hour TESOL training programme at an institution in Vietnam. This autoethnographic study analysed a TESOL teacher trainer's reflections on student teachers' engagement in the programme, using artefacts in the form of anonymous and open-ended short-text feedback from 30 students across approximately eight months. The reflection and feedback responses were thematically, anchored in a framework emphasising the three facets of in-class engagement, i.e., cognitive, emotional, and behavioural dimensions. Through the analysis, three themes were constructed: (1) theoretical-practical synthesis: how TESOL-related concepts are blended with concrete teaching examples; (2) pedagogical climate: the teacher educator's critical role in creating a receptive and engaging learning environment; and (3) professional pathway clarity: the student teachers' understanding of their future roles and trajectories in the TESOL arena. The research's findings provide TESOL educators with crucial insights into student teachers' engagement with teaching methods that synthesise both theoretical and practical components. Moreover, the research enriches the academic discourse, underscoring the potential of using the cognitive, emotional, and behavioural engagement triad as a diagnostic tool to understand students' learning experiences in TESOL training programmes.