English punctuation plays a crucial role in conveying meaning and clarity in written communication. Proficiency in punctuation is essential for effective language expression, especially for Vietnamese learners of English as a Foreign Language (EFL). This is particularly important for Vietnamese learners due to the significant differences in punctuation rules between Vietnamese and English. The use of correct punctuation marks ensures that ideas are organized coherently, facilitating comprehension for readers. Despite its significance, learners, particularly those at the intermediate level, may encounter challenges in mastering the intricacies of English punctuation. Therefore, this study was conducted to examine: (1) the familiarity of intermediate Vietnamese EFL learners with English punctuation, (2) the recurring punctuation mistakes in EFL written compositions, and (3) the underlying causes contributing to punctuation errors in the written work of Vietnamese learners. The study involved 41 participants, comprising intermediate EFL learners enrolled in an English Centre in the Mekong Delta, Vietnam. Data was collected through a punctuation proficiency assessment, essay submissions, and an interview. The findings revealed that the majority of intermediate EFL learners possess a moderately proficient grasp of English punctuation. Predominant punctuation errors encompass improper use of commas, periods, ellipses, and semicolons, as well as instances of missing semicolons, commas, and periods. The most frequently occurring errors are observed in semicolons, followed by commas and periods. Four overarching factors contributing to these punctuation errors include deficiencies in linguistic competence, limited knowledge of English punctuation rules, inattention to punctuation during the writing process, and a dearth of practice.