The rapid development of artificial intelligence (AI) is reshaping how English as a Foreign Language (EFL) teachers support vocabulary learning beyond formal classroom instruction. Existing studies have mostly examined learners' autonomous use of AI tools,while the teacher's role in mediating AI through everyday instructional planning remains under-theorized. This conceptual paper proposes a teacher-centered framework that explains how informal AI-mediated planning can enhance EFL vocabulary instruction. Synthesizing vocabulary acquisition theory, teacher cognition, learner engagement, and digital pedagogy, the framework links teacher decision-making to informal AI-supported planning, pedagogical mediation, learner engagement, and vocabulary development. The framework positions AI as a planning resource whose instructional value depends on teachers' professional judgment rather than as an independent instructional agent. The model provides a foundation for future empirical research and practical guidance forpedagogically grounded AI integration in vocabulary instruction.