The use of the students’ mother tongue in English during Foreign Language classes has been controversial for a long time. This research paper aimed to determining the students’ attitude and identifying main negative effects of lecturers’ use of mother tongue in English as foreign language classes at universities. This is a cross-sectional study on graduate-level students at five universities in Vietnam in October 2021. We administered a self-reported questionnaire to 385 graduate students on their attitude and practice towards the use of mother tongue in learning English from Vietnamese lecturers, and assess their perceived impacts on students’ speaking and listening skills. Descriptive analyses were used to summarize the results. Nearly all lecturers in the surveyed universities use first language in their lectures, half of respondents assumed that native language was used 20 to 50% of lecture’s time. Most students assessed the two main purposes for using first language were to convey grammar (27.8%) and difficult new words (25.8%) better. 60.0% of students agreed that it is better if both mother tongue and English are used by lecturers in English lessons. While more than half of the respondents disagreed that lecturers’ use of mother tongue impacted negatively on students’ pronunciation, 51% opposed that lecturers’ use of mother tongue limited students’ listening skills. Majority of foreign language classes in graduate level in Vietnam’s universities used both foreign and native language. Most students had a positive outlook on the impact of this dual-language system, however not being aware of the disadvantages of using mother tongue in teaching English to their speaking and listening skills.