Plagiarism is becoming more widespread in all types of educational institutions (Park, 2003). In Vietnam, this academic dishonesty has received little attention in teaching and training at the tertiary level (Hoang, 2014; Nguyen & Filipi, 2017; Nguyen, 2018; Tran, 2021; Phan, 2006). To bridge this gap, the current study aims to investigate Vietnamese students' perceptions of plagiarism in English academic writing before and after receiving plagiarism tutorials. Before the training, 44 sophomore students were asked to write an essay at home with the purpose of checking for plagiarism, and 10 students were randomly invited to the interview. The findings show that thirty-five out of forty-four students did plagiarize (30% to 90%), as reported by "Turnitin" software. The understanding of plagiarism among students was poor and they thought that plagiarism was not a big deal. They also did not know how to avoid plagiarism by using citations and quotations in writing. According to these findings, the content of plagiarism tutorials was determined and designed based on the standard of the American Psychological Association (APA). The content of crediting sources includes plagiarism introduction, types of plagiarism, crediting sources, and citing references. They discovered several misconceptions as well as new information about plagiarism after the training. It is suggested that language teachers should consider applying integration and interaction principles to the pedagogy to mitigate the prevalence of plagiarism in English academic writing. Furthermore, plagiarism tutorials such as seminars or workshops should be provided for all students from various departments to raise students' awareness of plagiarism in education.