<p><b>Task-based language teaching (TBLT) has been emphasised in second/foreign language education policy in many Asia-Pacific countries, including in Vietnam where, since 2008, official policy has mandated the use of language learning tasks in foreign language education. Consequently, Vietnamese teachers are expected to adopt task-based, communicative instruction in their classes. However, research to date in Vietnam has reported mixed success in teacher adoption and understanding of TBLT. The two-fold purpose of this study was to first investigate the influence of a textbook on task-based teaching, including how textbook tasks were interpreted and implemented by EFL teachers at a Vietnamese university, and, second, to investigate the impact of professional support on improving their engagement with TBLT. The research adopted an interpretive, qualitative, case study approach combined with Participatory Action Research (PAR). It tracked the use of tasks by three teachers across two research phases: a situation analysis followed by a PAR study. Data for the study included textbook analysis, classroom observations, stimulated recall interviews, semi-structured interviews with the teachers, and focus group interviews with students. </b></p> <p>Phase 1 investigated the relationship between the affordances for task-based teaching in the textbook New Cutting Edge, Elementary and teachers’ awareness of and uptake of these affordances. The communicativeness and task-likeness of activities and lesson sequences in the textbook were analysed and compared with the implementation decisions made by the teachers. Findings reveal that while the textbook has a high proportion of activities with low communicative value, many of these activities are either tasks or task-like. Data from classroom observations showed that, in their implementation of the textbook, the teachers consistently reduced the communicativeness and task-likeness of the textbook activities and replaced them with teacher-centred, explicit grammar explanation and drill practice. Interviews revealed the teachers’ rationales for their practices, including their belief that tasks are unsuitable for low proficiency students, pressure from exams, limited instruction time, and the teachers’ limited understanding of TBLT. </p> <p>In Phase 2, the three case study teachers participated in two PAR cycles aimed at helping the teachers to adopt TBLT. The PAR involved two professional development and learning (TPDL) workshops, collaborative planning of six task-based lessons (three lessons in each cycle), and subsequent teaching of these lessons. Data for Phase 2 included a descriptive account of the workshopping process, 18 classroom observations, semi-structured teacher interviews, and student focus group interviews. Findings revealed ways in which, over the two cycles, the teachers’ practices increasingly aligned with recognised principles of TBLT, although with each teacher moving along a unique professional trajectory in how they delivered the lessons and in their evolving understanding of TBLT and particular concerns. The study contributes to three intersecting fields of research: the implementation of TBLT in authentic classrooms; the relationship between teachers’ beliefs and TBLT practices; and TPDL for TBLT, an under-researched topic.</p>