In response to the global expansion in English-medium instruction (EMI) within business education, this study investigates its impact on self-efficacy among Vietnamese business students over 2 years. Drawing on Bourdieu's capital theory, data from 105 students were analyzed to explore the influence of socio-demographic factors. Surprisingly, EMI participation was associated with a negative change in self-efficacy, with no significant impact observed from social, economic, or cultural backgrounds. The findings suggest a mismatch between students' expectations and academic performance, emphasizing the need for tailored interventions to support self-efficacy development in EMI programmes.