Despite growing interest in using project-based language learning (PBLL) to foster active and authentic learning among English as a Foreign Language (EFL) learners, research on how students engage in PBLL, particularly with the support of artificial intelligence (AI) tools, and the role of AI in PBLL remains relatively limited. To address these gaps, this study examined how 291 Vietnamese EFL learners engaged cognitively, behaviorally, affectively, and socially in an AI-assisted PBLL environment. It also investigated whether these engagement dimensions and the use of AI tools affected students' perceived academic performance. A mixed-methods design was implemented, combining structural equation modeling of survey data with thematic analysis of semi-structured interviews. Results revealed that participants reported uniformly high engagement across all four dimensions and that these dimensions were significantly interrelated. Notably, affective engagement and AI tools emerged as two significant predictors of learners' self-reported academic performance, underscoring the pivotal role of emotional investment and AI integration in promoting language gains. Interview data further indicated that AI tools enhanced project efficiency and indirectly supported deeper engagement by freeing cognitive resources for creative and collaborative tasks. These findings highlight that integrating AI tools in PBLL can enhance learners' emotional engagement and overall project experience, which in turn positively influences their perceived academic performance.