In Vietnam, the approval of new English textbooks has heightened the demand for effective teacher training. This study uses a 6-point Likert-scale questionnaire to survey the perceptions of 103 secondary school EFL teachers regarding the effectiveness of the professional development (PD) courses they have participated in over a ten-year period. The study focuses on such aspects of PD effectiveness as professional attributes, subject matter knowledge, and the application of learned knowledge to teaching practice. The results reveal moderate overall effectiveness, with teaching knowledge showing the strongest gains and professional attributes the weakest. A significant non-linear relationship between teaching experience and perceived effectiveness emerged, with teachers in mid-to-late career stages (11-15 years and 21-25 years) reporting the highest benefits, while those with over 25 years of experience reported the lowest ratings. Although school type and gender did not show statistically significant effects, descriptive patterns suggested that teachers in rural normal schools perceived greater benefits than those in rural selective schools. A persistent gap was identified between knowledge acquisition and practical classroom application, particularly in adapting materials and teaching productive skills. The teachers expressed a strong preference for condensed, one-week PD sessions, highlighting the tension between time-efficiency and the need for sustained support. These findings underscore the necessity of differentiated, career-stage-responsive PD programs that balance practical intensity with ongoing support, address contextual needs, and strengthen the bridge between theoretical knowledge and classroom implementation.