This paper aims to explore how under authoritarian regimes, undergoing reform processes, divergent forms of environmental activism may emerge. Two severe cases of environmental degradation serve as our starting points: the marine disaster in the central coast of Vietnam in 2016 and the Mekong Delta's ongoing environmental degradation. While the former offers a case of rural grievances over mass fish death in Central Vietnam triggering protests on a national scale, the latter presents a continuum of environmental changes leading to serious impacts on deltaic livelihoods, albeit with no observable efforts of activism compared to the situation in other countries along the Mekong Delta. Drawing from in-depth interviews and participant observation with NGO workers in Vietnam who focus on environment and community development, we unravel the conditions, methods and rationalities behind their engagement (or lack thereof) with environmental activism in each case. We argue that the difference between the cases can be explained by tracing the process of politicising environmental grievances, taking into consideration culinary nationalism, anti-China nationalism and political opportunities under authoritarianism. Moving beyond current literature on activism under authoritarian regimes which relies mainly on institutional and/or social network approaches, our analysis helps further shed light on how contemporary environmental activism is mobilised in Vietnam from a geographically and politically grounded as well as culturally embedded position.