Despite being the poorest or second poorest participant, Vietnam outperformed all other developing countries, and many wealthier countries, on the 2012, 2015, and 2018 PISA assessments. We investigate Vietnam's strong performance, evaluating several possible explanations for this apparent exemplary achievement. After correcting for potentially non-representative PISA samples, including bias from Vietnam's large out-of-school population, Vietnam remains a large positive outlier conditional on its income. Possible higher motivation of, and coaching given to, Vietnamese students can at most only partly explain Vietnam's performance. The child-, household-and school-level variables in the PISA data explain little of Vietnam's strong PISA performance relative to its income level. At most, they explain about 30% of Vietnam's exceptional performance in math and reading. Further research is needed to understand the exceptional performance of Vietnamese students.