This explanatory sequential mixed-methods study investigated the impact of explicit culture-specific vocabulary (CSV) instruction on the intercultural communicative competence (ICC) and vocabulary retention of Vietnamese high school students. Quantitative data from a seven-week quasi-experiment showed that the experimental group significantly outperformed the control group on both vocabulary retention and ICC scenario assessments, yielding large effect sizes. Thematic analysis of qualitative data from teacher semi-structured interviews and student focus groups revealed that while the CSV framework enhanced student communication motivation and minimized foreign language anxiety, its classroom execution faced substantial challenges. These included rigid institutional curriculum pacing, high-stakes examination pressures, and textbook material deficiencies. The study concludes that explicit cultural contextualization is essential for real-world communication readiness, and calls for intentional reforms in EFL curriculum design and assessment practices.