This study explored participation by Japanese university English as a Foreign Language (EFL) learners in Computer Assisted Language Learning/Mobile Assisted Language Learning (CALL/MALL) vocabulary tasks through the lens of Complexity Theory (CT). CT, which studies how complex systems are influenced by changes in interconnected variables (Larsen-Freeman & Cameron, 2008), has been advocated as an approach that aligns well with Second Language Acquisition (SLA) and CALL research (Godwin-Jones, 2019). In this study, CT was applied to action research involving learners using smartphones to create game-based vocabulary quizzes. It aimed to understand how the context, personal attributes, and in-class interactions affected participation and learning outcomes. The results show the variability and nonlinearity of learners' language development, the importance of initial conditions, and the vital role of teacher input. It also demonstrated that there are various ways to successfully complete a task, so viewing tasks as dynamic systems may be a fruitful approach. [For the complete volume, "CALL for Widening Participation: Short Papers from EUROCALL 2020 (28th, Online, August 20-21, 2020)," see ED610330.]