Technology integration in second language acquisition (SLA) has become prevalent and prominent in the modern age. Quizlet has emerged as a vocabulary learning tool integrated not only into computers (CALL) but also into mobile phones (MALL), over the years; its effectiveness has already been proved via several quasi-experimental studies. However, such research is still limited; some did not even include a control group. Further, whether learners' individual differences may have any impact on Quizlet effectiveness in promoting lexical gains remains unknown. To address these gaps, this study was conducted with the participation of 68 Vietnamese undergraduates, following a pretest-posttest design together with semi-structured interviews. While the control group (N = 35) reviewed lexical items in a conventional way, the experimental group (N = 33) used Quizlet for vocabulary revision. Prior to the five-week treatment, all participants took the pretest, receptive and productive vocabulary knowledge tests, and three working memory tests. When all revision sessions were completed, all participants took the posttest. Then, five random learners in the experimental group were invited to join the semi-structured interviews. Results showed that while revision on Quizlet led to greater lexical gains, working memory and prior vocabulary knowledge did not play any significant role in predicting such improvements. Pedagogical implications for teachers and recommendations for future research were also provided to address vocabulary retention, individual differences, and the use of Quizlet more effectively.