In Vietnam, STEM-based teaching approaches have been recommended for schools since 2018 with the integration of multiple courses in schools. Teachers' perspectives on integrated STEM education, and how to design a STEM lesson that supports students' learning are critical for ensuring that an integrated STEM curriculum is to be implemented successfully. This paper aims to describe a study examining STEM education practices in teaching Mathematics, in particular "Trigonometry ratio of an acute angle" in Vietnam. This study utilized mixed methods approaches with three phases: the first stage employed a quantitative phase to examine Vietnamese teachers' perspectives on STEM education. The second phase was an experiment on teaching while applying STEM education to design and organize a mathematics lesson, and the third, final phase, was to employ a qualitative study to explore the Vietnamese students' experiences of STEM education connected with real-world problem-solving and 21st-century skills development. A total of 47 teachers and 85 students participated in this study and data was collected from multiple research tools such as questionnaires and semi-structured interviews. Findings showed Vietnamese teachers' perspectives on the necessity and importance of applying STEM education practices; however, they reported facing numerous challenges. A lesson designed as a STEM education application and the students' feedback in relation to real-world problems. Learning outcomes connected with 21st-century skills in this study were expected to provide further knowledge for further discussions and practices to enhance teaching and learning STEM and mathematics education in Vietnam.