The present study investigates the effect of task repetition and individual differences on Vietnamese English as a foreign language (EFL) learners' lexical use and fluency in oral task performance. The study adopts a within-subjects design with forty students performing the same narrative task twice. Students also completed two prior vocabulary knowledge tests (i.e., a receptive size test and a productive level test) and two working memory tests(i.e., a backwardspan task and a Vietnamese-version operation-span task). Lexical complexity was measured in two aspects (i.e., lexical sophistication and lexical diversity). Fluency was assessed in terms of articulation rate. Unlike previous studies, our study surprisingly did not reveal an effect of task repetition on lexical complexity and fluency. Interestingly, prior vocabulary knowledge and working memory appeared to be good predictors of learners' lexical complexity and fluency.