This study investigates how explicit vocabulary support influences engagement and lexical learning in paired interactive information-gap tasks. Sixty-six Japanese EFL learners performed the tasks in either a vocabulary-support condition, with access to task-relevant collocations, or a task-only condition. Engagement was operationalized using discourse-analytic indicators from transcribed interactions and summarized via principal component analysis, resulting in three engagement patterns reflecting (1) target lexical item use, (2) joint meaning making, and (3) sustained interactional progress. Bayesian regression and mediation analyses showed that vocabulary support created meaningful differences in these patterns; however, the patterns did not mediate its effect on gains in explicit productive knowledge of the target forms. This suggests that interactional engagement may relate to aspects of lexical learning not captured by written productive tests. This study illustrates how a task-driven composite approach can retain the visibility of engagement indicators, supporting interpretability in relation to task design and classroom practice.
