This inquiry explored the under-explored phenomenon in Vietnamese tertiary settings of code-switching practised by EFL (English as a foreign language) teachers in classroom instruction, as well as their awareness of this practice. Among the foreign languages taught and learned in Vietnamese universities, English is the most popular. The research design involved data-driven assay of 12 teachers’ code-switching behaviour in the EFL classroom from four different main sources of information: classroom observations, class recordings, interviews with the observed teachers, and field notes. The findings display that teachers used code-switching very commonly in their English instruction, for both pedagogical and affective reasons. Code-switching, when done deliberately and selectively, can be a positive strategy to use in the EFL classroom, but not if done habitually and automatically. Since code-switching is a natural occurrence amongst bilingual speakers, teacher training programs may need to include code-switching as a deliberate teaching strategy. In addition, before any government policy or guidelines on code-switching are introduced, teachers need specific training on strategies for its effect use. The interviews with the Vietnamese university EFL teachers disclosed key reasons for their code-switching in the EFL classroom, which are similar to those found in other studies. These are explored with a pedagogical and affective focus.