This study examined the effects of involvement load-based tasks on vocabulary learning in a foreign language, as well as the extent to which task effects are predicted by learners' metacognition (i.e. metacognitive knowledge and regulation). A total of 120 Chinese university students of English as a foreign language (EFL) were randomly assigned to four task conditions: (1) reading; (2) reading + gap-fill; (3) reading + writing; and (4) reading + writing with the use of a digital dictionary. The Vocabulary Knowledge Scale was adapted to measure condition effects. The Metacognitive Awareness Inventory was used to examine learners' metacognitive knowledge and regulation. Results revealed that the group of learners who completed reading + writing tasks with the use of a digital dictionary demonstrated the best performance in acquiring receptive and productive vocabulary knowledge, followed by the reading + writing group, the reading + gap-fill group, and, finally, the reading only group. Multiple regression analysis supported the predictive effects of metacognitive regulation on task-based vocabulary learning. Structural equation modelling presented an overall profile of task-based vocabulary learning and metacognition. Based on the findings, we proposed a framework to understand the relationship between learners' metacognition, task type, and L2 vocabulary learning.