Scholars concerned with language policies (LPs) argue that globalization has brought about various political, socio-economic, and linguistic shifts that increasingly impact on teacher agency (e.g., Zhao & Baldauf, 2012). Recently, the LP literature has increasingly acknowledged the agentive role of teachers as a critical factor in implementing policy (Liddicoat & Baldauf, 2008). These and other scholars argue that teachers have the potential to exercise their transformative roles in implementing and responding to the LP changes (Menken & Garcia, 2010). However, agency has remained under-examined (Hamid, Nguyen, & Baldauf, 2013; Ramanathan & Morgan, 2007; Zhao & Baldauf, 2012) especially in developing settings like Vietnam. In the context of national English language education policy reforms in Vietnam, the need to explore teacher agency in response to the reforms to better understand the policy implementation process at the local level is critical. Using Fullan's (1993) theory on change agency, this paper examines how a group of English teachers in a remote mountainous area in Vietnam interpret, interrogate, and appropriate the current English LP. Data were collected from interviews and classroom observation of the teachers to shed light on their visions and practices in responding to the reform policy. The findings indicate that teachers are highly capable of exercising their agency as comprehensive policy implementers. They do so by resisting the poorly regulated English LPs and working towards meaningful pedagogical transformations. The study thus emphasizes the urgent need for a comprehensive understanding of teachers' agency in LP decision-making and implementation. It also makes a contribution to explicitly theorizing the concept of teacher agency from the perspective of Fullan's theory on change agency. Lastly, the study considers implications for teachers, policy-makers, and school leaders to support the active roles of the teachers in reform implementation.