During COVID-19 lockdowns, Vietnamese universities, like elsewhere, were compelled by social distancing rules to transition from on-site to online education, including examinations. However, many of these universities were not prepared for such sudden and drastic changes. This situation raises questions about if and how the students' cheating behaviors were affected. Based on an online survey of 384 students and 99 instructors from three universities in Vietnam conducted in 2021 during the peak of the pandemic, we present empirical evidence of the perceived increase in the prevalence of cheating behaviors when universities moved to online exams. Such an increase was highest in time-limited individual tests. Among the different cheating behaviors, the use of search engines and the use of prohibited materials, as well as plagiarism, were the most common. Our data also indicates a reverse correlation between the students' negative attitudes toward cheating and the perceived prevalence of such behaviors. The findings, combined with a comparison between the observations and attitudes of lecturers and students, provide potential policy implications for the universities on how to address online cheating.