<p>This study explores how Chinese as a Foreign Language (CFL) learners cognitively and affectively engage in vocabulary learning within a Seamless Chinese Vocabulary Learning (SCVL) environment. Guided by a design-based research (DBR) methodology, it adopts a qualitatively driven mixed-methods approach to examine engagement as a dynamic learning process over a 16-week instructional intervention. Thirty-two intermediate CFL learners participated in a mobile-assisted SCVL design using DingTalk, which connected formal classroom instruction with informal, real-life vocabulary use. Drawing primarily on qualitative evidence from interviews, open-ended questionnaires, classroom observations, technology usage records, and learner-generated artifacts, the study employs thematic analysis to trace how learners’ cognitive engagement and affective experiences evolved during Chinese writing activities. Descriptive quantitative information, including lexical diversity indices and post-intervention survey responses, served a complementary role by documenting changes in vocabulary use across instructional stages. The findings show that learners’ vocabulary engagement evolved through iterative cycles of contextualized practice, feedback, and revision, accompanied by fluctuating affective experiences such as confidence, motivation, anxiety, and fatigue. At the same time, the analysis reveals cross-cutting pedagogical and contextual challenges, including classroom noise, technical disruptions, digital literacy constraints, and peer feedback fatigue, which shaped learners’ engagement trajectories over time. Across instructional stages, learners demonstrated increasingly flexible and context-sensitive vocabulary use in their written artifacts. By foregrounding learner engagement as situated, evolving, and emotionally mediated, this study offers fine-grained, empirically grounded insights into the design and enactment of seamless vocabulary learning for CFL writing instruction.</p>