This study investigated to what extent quantity (frequency/amount of time) and quality (diversity) of informal digital learning of English (IDLE) activities was conducive to second language (L2) vocabulary outcomes. Data (n = 77) were gathered quantitatively and qualitatively through a questionnaire, English vocabulary tests (ie, measuring receptive and productive vocabulary knowledge), and a semistructured interview from three universities in an English as a foreign language Korean context. Contrary to earlier findings, this study found that quantity of IDLE was not related to vocabulary scores. It revealed that the quality of IDLE was significantly, positively associated with vocabulary outcomes. These results lead us to the conclusion that the frequent practice of IDLE activities may not automatically guarantee successful L2 vocabulary gains. Rather, engagement with varied types of IDLE activities that combines both form- and meaning-focused language learning is essential for achieving L2 vocabulary acquisition.