This qualitative study investigates the ethical dimensions of English as a Foreign Language (EFL) teaching in the Vietnamese context. Focusing on a sample of fifteen Vietnamese EFL teachers across different career stages—novice, mid-career, and near-end—the research employs semi-structured interviews. The study is underpinned by Constructivist Grounded Theory and Cultural-Historical Activity Theory, providing a framework for understanding the dynamic interplay between individual experiences and the broader cultural and educational context. Three key themes emerged from the thematic analysis: interplay of cultural values and ethical teaching, ethical dilemmas in language instruction, and evolution of moral identity in teaching. These themes reveal how deeply ingrained cultural values, particularly those influenced by Confucian traditions, shape EFL teaching practices in Vietnam. They also highlight the widespread nature of ethical challenges and how teachers’ moral identities evolve throughout their careers. The study’s findings have significant implications for teacher training, professional development, institutional culture, and policy-making in the field of language education.