Information about the Vietnamese physical education (PE) programme is limited due to a lack of relevant studies. Using the Health Optimising Physical Education as a conceptual framework, this study examined students' physical activity (PA) levels in the fifth-grade PE programme in Vietnam and barriers to programme implementation. Eight schools (28 PE classes) were randomly selected for assessing students' PA levels, lesson context, and PA promotion during PE lessons using the System for Observing Fitness Instruction Time. In-depth interviews were also conducted with school administrators, PE instructors, and classroom teachers using semi-structured guides to examine barriers influencing the implementation of the current PE programme. Results showed students spent 33% of PE time in moderate-vigorous physical activity (MVPA), which is less than the international recommendation of 50%. The amount of MVPA varied by school type, instructor gender and qualification. Programme implementation was negatively affected by the current structure that limits autonomy, a proclivity for a "one size fits all" approach to teaching PE, and limited motivation for PE instructors to improve their teaching. The low perceived value of PE, lack of support and professional development for existing PE instructors, and an over-emphasis on sports training due to available financial incentives were also identified as important barriers that affected programme implementation. Addressing these barriers could help improve the quality of the Vietnamese PE programme.