Effective business communication in English has become essential for university graduates in English as a second language contexts as global workplace demands grow. This study examined the integration of project-based learning into a 15-week Business Communication course at Foreign Trade University, Vietnam, addressing three research questions: (a) whether students’ business communication proficiency improved through project-based learning, (b) how students perceived the learning process, and (c) what challenges the instructor faced during implementation. Participants were 60 third-year undergraduates majoring in Business English, organized into 12 teams of five. Using a convergent mixed methods design, the study collected four data types: pre- and post-course rubric-based performance assessments in three skill areas (business writing, oral presentation, negotiation); a 30-item Likert-scale questionnaire (n=60); three focus group interviews with 22 student volunteers; and a semi-structured interview with the course instructor, supported by classroom field notes. Quantitative analyses revealed statistically significant within-subjects gains across all three skills (Cohen’s d z = 1.36-1.61, p < 0.001). Questionnaire data indicated broadly positive student perceptions, with 82-89% agreement across the six measured dimensions. Focus group data converged on four themes: increased ownership, real-world language use, interpersonal-skill development, and a time-bounded period of initial uncertainty. The instructor reported greater student engagement and interaction than in previous teacher-led semesters, alongside substantial scaffolding demands during early project phases. The findings provide empirical support for project-based learning as a contextually relevant approach for advancing business English proficiency in TESOL programs at internationally oriented Vietnamese universities.