This article examines the use of prompt-based lexical collocations in opinion essays by Vietnamese English as a foreign language (EFL) students. Fifty second-year English majors at a Vietnamese university wrote 100 opinion essays on two topics as progress tests. The AntConc programme (Anthony, 2020) was employed to identify the frequencies of use of the prompt-based lexical collocations in the essays. Paraphrases of the target lexical collocations were further identified and calculated for their occurrences. The AntConc concordance lines that target the prompt-based collocations or paraphrases were analysed for errors, with the consultation of the Oxford Collocations Dictionary (OCD), the British National Corpus (BNC), and the Corpus of Contemporary American English (COCA). The results revealed that students repeated the source lexical collocations and replaced them with various word combinations, though recycling occurred to a greater extent. Erroneous use involved use of i) incorrect word combinations, mainly through wrong choice of verbs, adjectives, or nouns that do not collocate with adjacent words and ii) incorrect grammatical forms of the individual lexical words of the target collocations (i.e., omission of plural marker --s, misuse of parts of speech). Pedagogical implications are discussed to assist students in using lexical collocations appropriately in L2 writing.