English Medium Instruction (EMI) has become increasingly common in higher education as universities internationalize curricula and expand students’ access to global academic knowledge. However, learning disciplinary content through English remains challenging for students in contexts where English is not their first language. While previous EMI research has focused on language proficiency, policy, learner attitudes, and lecturer practices, less attention has been paid to the role of peer interaction in supporting content understanding. This qualitative-dominant mixed-methods study addresses this gap by examining how students use peer interaction as a process of collaborative meaning-making in a Vietnamese university EMI course. The study involved 24 second-year students observed across three EMI lessons; 16 students in eight pairs were audio-recorded during content-focused pair tasks, and 10 students participated in semi-structured interviews. Data were analyzed thematically, supported by descriptive counts of interactional moves. Findings indicate that peer interaction served four main functions: clarifying task requirements and disciplinary concepts, providing explanations and paraphrasing, strategically using Vietnamese to secure conceptual understanding, and jointly constructing responses. Students also perceived peer interaction as both cognitive and affective support, helping them understand content and participate more confidently in class. The study argues that peer interaction should be recognized as an important mediational resource in EMI pedagogy rather than merely a peripheral communicative activity.