Collocational development is of great significance to second language acquisition. Among different types of collocations, adjective-noun collocations are notoriously difficult to EFL learners, but there has been limited research, especially in Vietnam, on this type of collocations in the field of teaching and learning English. To address this issue, the current study, employing the quantitative approach, investigates Vietnamese university students’ receptive and productive knowledge of adjective-noun collocations and the relationship between their language proficiency and their collocational knowledge. Results reveal that the difference between the students’ receptive and productive knowledge was more significant when they reached higher levels of English, and even those at an advanced level had great difficulties in identifying erroneous collocations and using them correctly. The number of collocational errors appeared to increase with rising proficiency. Results also indicate a close relationship between language ability and collocational knowledge, thereby emphasizing the importance of teaching collocations explicitly in English classrooms.