Abstract Previous studies have discussed stakeholders’ perceptions of using first language in English-medium instruction (EMI), but only a few have examined classroom practices in Vietnam. Against this backdrop, this article focuses on lecturers’ translanguaging practices in an EMI programme where all the lecturers and students were Vietnamese. More specifically, it employs the conceptual framework of ROAD-MAPPING, proposed by Dafouz and Smit (“The ROAD-MAPPING framework: Taking stock and paving future directions for English-medium research”, in Emma Dafouz and Ute Smit (eds.), Researching English-medium higher education: Diverse applications and critical evaluations of the ROAD-MAPPING framework, 1–19. London: Routledge, 2016), to see if there was substantial evidence of a constructive co-existence between English and Vietnamese – the first language – in this EMI programme. The findings suggest that under the lecturers’ and students’ agency, the two languages were distributed across various domains of classroom practices to benefit students’ learning of content knowledge. In teacher talk, English and Vietnamese have different functions to support the process of knowledge co-construction. The study implies that the current monolingual orientation promoted in many top-down EMI policies should be reconsidered, given the linguistic diversity of higher education settings. In other words, EMI should be promoted as an educational environment where students can access their linguistic repertoires and develop their academic knowledge bi/multilingually. Translanguaging, therefore, should be acknowledged as an inclusive pedagogical practice.