This study aims to analyze the determinants of Net Interest Margin (NIM) of Vietnamese commercial banks, based on 200 observations from 20 listed banks during 2015–2024 and estimated using the System GMM method. The results indicate that the non-performing loan ratio (NPL) has a positive effect on NIM, while bank size (SIZE), loan exposure (LOAN), and liquidity (LIQ) have negative effects, reflecting competitive pressure, high funding costs, and excessive liquidity reserves. The COVID-19 pandemic reduces NIM, but the NIM × COVID interaction variable shows that banks can leverage previous-period NIM to mitigate the impact, highlighting the importance of internal governance and risk management strategies. The study suggests that banks should optimize size and capital structure, manage credit risk and liquidity efficiently, and develop flexible strategies to respond to economic shocks, thereby maintaining stable NIM and enhancing competitiveness...