In this paper, we report on an international collaborative project designed to address the professional development needs of Vietnamese teachers for the implementation of Content and Language Integrated Learning (CLIL). We collected data on a group of CLIL Vietnamese teachers and leaders through online interviews. Drawing on Cultural-Historical Activity Theory (CHAT), the paper illuminates how CLIL teachers, as the subject of the professional learning (PL) system, utilized tools to develop their capacity for CLIL teaching against the backdrop of various sociocultural factors that regulate this learning process. The analysis helped us to identify various challenges facing CLIL teachers' PL. These challenges arose from an absence of an effective learning system mostly related to contextual obstacles and widespread misconceptions, even among the leaders and CLIL trainers, about the nature of CLIL. This study contributes suggestions to enhance the quality of PL for CLIL teachers, which is a pressing issue for the successful implementation of CLIL in the context of Vietnam and beyond.