This study investigated the impact of synchronous computer-mediated communication (SCMC) mode and familiarity with partners on learner engagement in second-language task-based interaction, and whether learner engagement is linked to subsequent joint-written-text quality. Ninety-eight Vietnamese learners of English were assigned into (±) familiar groups and performed a picture-sequencing tasks in 2 SCMC modes (i.e., video and text chat). Scores of 3 types of learner engagement (cognitive, social, and emotional) were compared across the conditions. Results showed that scores of all engagement types in the video chat were significantly higher than in the text chat. Familiar dyads also showed higher engagement than unfamiliar peers during the interaction. Learners reported different reasons for their preferences of video chat over text chat. Language-related episodes, semantically engaged talk, and mutual help as measures of learner engagement were predictive of the subsequent text quality. The results contribute to the general understanding of the characteristics of video and text chat and their impact on learner engagement and text quality.