The study looked at factors that influenced the self-efficacy in teaching English as a Foreign Language (EFL) of a group of university teachers in Vietnam. Previous studies yielded contradictory results regarding the sources of self-efficacy information. Very little empirical research on the potential role of cultural factors on self-efficacy and on teachers’ self-efficacy in EFL contexts has been done. Researchers disagree on whether participating in a new setting can enhance teachers’ self-efficacy beliefs. Therefore, this study explored the relationship between Vietnamese teachers’ discourses of effective teaching practices and their self-efficacy beliefs, the influence of Vietnamese culture and context on teachers’ self-efficacy beliefs, and whether participating in the research led to a change in the self-efficacy beliefs of the teacher participants and of myself as researcher. The research took the form of a qualitative case study. Participants were eight university teachers of the English language at a technical university in Vietnam. Data collection lasted six months. Data collection tools included focus group discussions, individual interviews, journaling, and observations. An inductive coding process and thematic analysis were used for analysing data. Findings indicate that social persuasion was the most influential source of self-efficacy information. The study shows that different sources of self-efficacy information interacted with one another to influence the two dimensions of self-efficacy. Besides, it appears that teachers’ understanding of a number of environment and workplace factors appeared to constrain some teachers into adopting the Grammar Translation Method (GTM) approach and possibly reduced their self-efficacy in adapting a Communicative Language Teaching (CLT)-oriented approach. After participating in the study, the teaching approaches of study teachers and my own approaches seemed to be more CLT-oriented although most of us were characterised by a low sense of self-efficacy in adapting this approach in the Vietnamese classrooms. I also developed an enhanced sense of self-efficacy in doing the thesis. Findings suggest that several aspects of Vietnamese culture, e.g. the concept of face, are likely to have influenced the way the study teachers selected, weighted and interpreted efficacy-building information. There were certain features of context, e.g. the state of leadership practices, which may have affected what constituted sources of self-efficacy information and how they operated. In addition, it is plausible that changes in context, e.g. teaching different kinds of students, led to a change in the way the teachers and I weighed and selected self-efficacy information. Self-reflection, self-doubt and self-regulation were other factors causing fluctuations in the study teachers’ and my self-efficacy. My study contributes to a widening understanding of how different aspects of culture can impact on self-efficacy. It provides examples to challenge the claim that the self-efficacy of experienced teachers is stable and the widespread view that a negative sense of self-efficacy induces individuals to give up and make less effort. The study shows the relationship between teacher self-efficacy beliefs and their discourses of EFL instruction, i.e. their self-efficacy in using different aspects of a communicative approach fluctuated at different stages of the study. The study points to the need to improve leadership practice and teaching conditions at the faculty and university. Preparing teachers for regulation strategies, encouraging them to work collectively, and offering more professional development programs are likely to develop a stronger sense of self-efficacy among teachers. It would be useful if future research could focus more on classroom observations to avoid the reliance on self-report data. More studies on the influence of culture on teacher self-efficacy with an inclusion of scales to measure different cultural factors are needed. Longitudinal studies are desirable in understanding changes in teacher self-efficacy under the influence of context.