The implementation of content and language integrated learning (CLIL) has been enforced by many education systems across the world as a means of improving their foreign language education or strengthening multilingualism in their society. CLIL in the context of Vietnam has recently been promoted as a means of tackling the country's ineffective English language education and upholding its science, technology, engineering, and mathematics fields. This study was conducted to gather information on the teaching of mathematics in English in Vietnamese high schools. Data were collected from a survey and follow-up interviews at 42 high schools across the country. The survey involved 39 school administrators, 78 teachers, and 500 students who had direct experience with teaching and learning mathematics in English while the follow-up interviews were conducted with 14 school administrators and 35 teachers. Statistical analyses performed indicated that CLIL-practising schools in Vietnam performed rather satisfactorily in terms of assessing CLIL learning outcomes and using content and pedagogies appropriate for their CLIL objectives and their school context. CLIL mathematics activities were perceived to be less satisfactory than other CLIL teaching and learning aspects due to practical conditions that constrained the range and diversity of CLIL mathematics activities. Vietnamese schools reported tailoring their CLIL mathematics teaching and learning to suit their particular school settings, which could be seen appropriate at the current early stage of CLIL mathematics implementation in Vietnam. The heavy dependence on foreign-imported curriculum and textbooks, however, still posed challenges in creating meaningful learning outcomes for CLIL students in Vietnam.