This study explored the extent to which discourse-level errors affected band scores in IELTS Writing Task 2 essays written by Vietnamese English learners. The primary objective was to identify how errors related to coherence, cohesion, idea development, and paragraphing influenced performance in the criteria of Coherence and Cohesion and Task Response. A total of fifty Task 2 essays were collected from test-simulated writing sessions and scored independently by certified IELTS raters using the official public band descriptors. Discourse-level errors were manually identified and classified using a customized taxonomy developed for this study. The analysis revealed typical manifestations of these errors, such as poor transitions, illogical flow of ideas, and inconsistent paragraph structure, with inconsistencies evident in aspects including missing topic sentences, mixing main and supporting ideas within paragraphs, or paragraphs that failed to adhere to the topic introduced in the opening sentence. The study highlighted the critical impact of discourse competence on academic writing, suggesting that writing instruction should prioritize the development of logical organization and argumentative progression over a sole focus on correcting grammar and vocabulary.