Action research (AR), which involve rigorous cycles of implementing and reflecting on educational changes, brings genuine values to language teaching. However, due to several factors related to school cultures and the teacher-researchers, AR does not always live up to its expectations. Generally, previous studies have steered towards instigating an awareness of AR at the expense of obtaining empirical evidence on competencies and conditions for AR. This study examines teachers’ action research competencies and challenges in doing AR. A questionnaire (70 items, α = 0.809) was administered to 105 EFL teachers in various high schools in Vietnam. Among these participants, eight partook in follow-up semi-structured interviews. Chi-square analysis confirmed that top-rated competencies aligned with the traditional roles of high school English teachers, including giving feedback, using visual aids for presentations, and self-assessment. In contrast, teachers were least sure of their research skills and techniques. Thematic analysis complemented the quantitative data, ascribing the underdeveloped research capacity to a lack of guidelines, motivation, and resources. Consequently, the practice of AR might be improved with the cultivation of a teacher research culture. Other measures to incentivize high school EFL teachers in research engagement are also discussed in this paper.