Following the international language proficiency standards, Vietnam has recently set the advanced English proficiency (C1- CEFR) for teacher graduates from an English teacher education program. Considering regional differences, this standard setting has raised a concern about its feasibility. Therefore, this paper aims to report the language proficiency development of English teacher trainees at a Vietnamese university as an illustration and examine training factors that may influence this proficiency outcome. To this end, a mixedmethod approach is used in which a self-report questionnaire is designed to collect the data on the trainees’ self-assessment of EP and their responses to the training factors. Also, a group interview is then conducted to obtain qualitative evidence that backed up the questionnaire results. The participants are 41 fourth-year EFL teacher students, seven of whom agreed to participate in the group interview. The results show that approximately 62.4% of the candidates selfassessed their level equivalent to the C1-advanced proficiency, while a significant percentages till desired additional language proficiency support. Their responses to the training factors reveal that they are overall satisfied with the effectiveness of English proficiency training. Noticeably, their proficiency scores positively correlate with their use of self-regulated learning strategies, which in turn correlated with the types of teaching and assessment activities during the training years. Their starting proficiency level also has a significant correlation to their proficiency gain at the end. These findings offer useful implications for preparing foreign language teachers in the context towards the general proficiency standards, and for understanding expectations versus realities regarding this issue.