Many policy changes in teaching and learning English in Vietnam have been implemented. To make those changes successful, the quality of English teachers is paid remarkable attention to by stakeholders. In particular, the quality of their teaching is said to be the determining factor in the success or failure of the changes. Since then, many professional development programs for English teachers have been implemented in Vietnam. This research explores Vietnamese teachers’ satisfaction with the effectiveness of a large-scale general English proficiency training program provided by MOET’s National Foreign Languages Project for school teachers in the Mekong Delta. Employing questionnaires with 174 school teachers in the Mekong Delta, the study sheds light on the main factors that these teachers perceive as crucial factors that influence the quality of the training program and their satisfaction. Findings from the survey reveal that critical factors impacting the training effectiveness include training materials, training duration, teaching and learning facilities, qualified trainers, and support from management. These findings provide a solid foundation for proposing an efficient model training program to effectively support English language teachers in the Mekong Delta to accomplish the required English proficiency level.