This study examines ChatGPT as a supportive tool for an ESP lecturer teaching first-year business students (B2-C1 proficiency, majors in Banking and Finance, Accounting, Business Administration, Logistics and Supply Chain Management) at Foreign Trade University, Vietnam. The tech-supported ESP course was delivered primarily via Microsoft Teams. The mixed-methods case study explores opportunities and challenges of integrating ChatGPT in teaching methods and assessment design (e.g., diverse question generation, formative feedback), alongside its potential influence on student outcomes in CALL contexts. Data were collected from 81 students, including Microsoft Teams interaction logs and materials, lecturer observations and reflective notes, student reflections, an open online quiz (n=50), and a final exam (n=50). Descriptive statistics showed quiz mean scores of 18.86/30 improving to 37.46/50 on the exam (average 37.2% gain). Qualitative findings highlighted ChatGPT's role in bridging content gaps, fostering critical thinking, and enabling personalized support, but also concerns regarding overreliance and ethical issues such as balanced use and academic integrity. While preliminary results suggest thoughtful integration of ChatGPT can enhance ESP instruction, the absence of control groups limits causal claims about learner progress. Future iterations will incorporate comparative cohorts. This case offers practical insights for practitioners navigating AI's opportunities and limitations in classrooms.