This paper assesses and compares the prevalence of plagiarism across different student and assignment characteristics at a university in Vietnam, using the similarity index reported by the text-matching software Turnitin as a proxy measure of plagiarism on a sample of 681 student papers. The findings present a level of match higher than reported in earlier studies at universities outside of Vietnam, with an average similarity index of 29.06%. Controlling for the gender and major of the students, the extent of plagiarism is implied to be negatively correlated with the students' academic performances and with the likelihood of being caught, and positively correlated with the length of the assignments. Thus, this study, relying on actual text-matching data rather than self-reported surveys, provides the first empirical test of two theoretical plagiarism models proposed in the literature. The explanatory factors confirmed by this study illustrate the potential benefits of the use of software tools to detect plagiarism and can help refine academic integrity policy formulations for universities.