Providing teachers with adequate professional development (PD) is a central tenet to enhance education quality. In Vietnam, despite the blossoming of PD activities promoted over the past decade, the central question of how effectively these existing activities facilitate changes in teachers’ practice has been under- researched. This mixed-method study responded to the scarcity in understanding the effectiveness of PD activities in the Vietnamese setting by employing a questionnaire administered to 80 high school teachers and six semi-structured interviews. Evidence from the questionnaire and interviews revealed that EFL teachers participated in PD activities on an occasional basis. Institution-internal or in-house professional activities were most common, while joining a professional affiliation such as a TESOL association was the rarest. Also, PD activities have positively reinforced the teachers’ language proficiency, teaching practice, and planning practical lessons to meet students’ learning needs. The discussions and recommendations are made for enhancing the quality of PD activities.