Prior to enrolling in an English as a medium of instruction (EMI) institution, students must show an English proficiency level through meeting a benchmark on a standard English proficiency test, which is typically aligned with the Common European Framework of Reference for Languages (CEFR). Along with overall English proficiency, aural/written vocabulary level mastery could also predict students’ success at EMI institutions, as students need adequate English vocabulary knowledge to comprehend lectures and course readings. However, aural/written vocabulary level mastery has yet to be clearly benchmarked to CEFR levels. Therefore, this study aimed to investigate the correlations between students’ aural/written vocabulary level mastery and their CEFR levels. Forty undergraduate students in a Macau EMI university were recruited to take one English proficiency test and two vocabulary level tests (i.e., Listening Vocabulary Levels Test (LVLT) and the Updated Vocabulary Levels Test (UVLT)). Correlation analyses were conducted to explore the relationship between students’ CEFR levels and their mastery of listening and reading vocabulary levels. A positive correlation was found between students’ CEFR levels and their mastery of receptive aural vocabulary levels (ρ = 0.409, p = 0.009). Furthermore, a statistically significant positive correlation was found between students’ CEFR levels and their mastery of receptive written vocabulary levels (ρ = 0.559, p < 0.001). Although positive correlations were observed, no clear pattern was identified regarding the relationship between students’ CEFR levels and their mastery of aural/written vocabulary levels. Regression analyses were further conducted to determine the extent to which the combination of receptive aural and written vocabulary knowledge predicts the CEFR levels. The results indicated that the regression model that included only UVLT scores better predicted the CEFR levels. Given the positive correlations observed between students’ CEFR levels and their mastery of vocabulary levels, this study’s findings suggest the inclusion of aural/written vocabulary levels as additional indicators for ensuring student academic success in EMI institutions. Implications for EMI universities on student admissions, classroom teaching, and provision of additional English courses were provided.