The use of mother tongue to teach a second language has been a controversial issue in the field of second language acquisition. This study aims to compare the effectiveness of using mother tongue (Vietnamese) to teach a foreign language (English) under the pressure of the implementation of an English-only policy at the tertiary level in Vietnam where English is taught as a foreign language. The mixed-methods approach was conducted to contrast the perspectives of 11 teachers participating in semi-structured interviews, and 255 students answering 5-point Likert scale questionnaires. These research instruments were carefully designed by the researchers adopting the factual and behavioural criteria proposed by Dörnyei and Taguchi (2009). The results show that the minimal use of Vietnamese is utilized as a necessary pedagogical tool to teach English. The empirical results propose that the English-only policy can be effective if the actions such as more English learning time, English entry tests, English materials and teaching aids with techno-facilities are seriously taken into consideration. To adopt the English-only policy in EFL classes, school administrators have to renovate the foreign language regulations while teachers must adapt their teaching methodology to meet the requirements of the learner-centered approaches.