English medium instruction is a globally emerging phenomenon and countries have recorded an exponential growth in EMI courses in the past few decades. Studies examining EMI have been conducted in various contexts, especially in higher education sector, posing its beneficial language and content outcomes. The implementation of EMI programs in Vietnam, however confronts a number of problems and constraints. This study attempts to figure out challenges facing teachers in applying English as medium instruction in teaching core curriculum subjects in National Economics University. The study also aims to investigate teachers' strategies to cope with difficulties, especially their adaptation to teach online EMI courses during the Covid-19 pandemic. Findings from the study shed some light for positive improvements in EMI programs offered in higher education. Possible implications will also be proposed to EMI curriculum developers and education policymakers for future effective development of EMI tertiary programs in blended-learning context.