Various explicit reading support cues, such as gloss, QR codes and hypertext annotation, have been embedded in e-books designed specifically for fostering various aspects of language development. However, explicit visual cues are not always reliably perceived as salient or effective by language learners. The current study explored the efficacy of implicit reading support cues--cues that are imperceptible to second-language (L2) readers during their L2 digital reading--for promoting L2 vocabulary acquisition. Results suggest that subliminal formal priming--being one type of implicit reading support cues--helped L2 readers significantly improve their form-meaning vocabulary knowledge through e-book reading. In particular, subliminal formal priming was more effective when the digital content, including the text and relevant illustration, was presented to L2 readers simultaneously, rather than incrementally. The results have important implications vis-à-vis the need for the inclusion of implicit reading cues, and the optimal digital input presentation mode for enhancing L2 vocabulary gains.