The current study investigated factors that predicted variation in the English proficiency of a cohort of 75 students of English enrolled in an English language teacher training program at a Vietnamese University. The study, conducted after two years of implementing the program, used a practice IELTS test to determine the proficiency levels and a questionnaire of influential factors as well as an interview to probe the factors. The results showed that learner factors including entry level, time management for part-time job and self-study, and use of self-regulated learning strategies could better predict their English proficiency than others related to direct training such as assessment and teaching methods.