Content and Language Integrated Learning (CLIL), widely established as a mainstream approach in ESL countries, is gaining traction in EFL contexts through selective programs and initiatives. Reflecting this trend, Vietnam's educational reforms under the 2018 General Education Program have incorporated CLIL-informed lessons into newly adopted English textbooks, signaling a step toward integrating language and subject learning. However, the CLIL-informed lesson remains relatively new to EFL teachers in Vietnamese high schools. Surveying EFL teachers' understandings and their teaching practices are crucial to discover their theoretical knowledge and practical application in teaching CLIL. It is essential to examine how other factors influence this relationship and identify the challenges and support teachers need in delivering these new lessons. The study employed a mixed-methods approach aimed at investigating EFL high school teachers' understandings, teaching practices, challenges and needs when delivering CLIL-informed lessons. The study collected quantitative data from 326 participants, along with qualitative data from interviews from 6 chosen teachers. Findings revealed that with differing cognitive factors, educational backgrounds, and working circumstances, teachers demonstrated their adaptability by tailoring the lessons with their own "localized CLIL model" and expressed further support in training for maximizing CLIL benefits in harmony with other contextual constraints. These findings, under the lens of Cultural-Historical Activity Theory, serve as valuable references for other EFL teachers, curriculum developers and textbook publishers in supporting the implementation and diffusion of CLIL.