Extensive Reading (ER) emerges as a promising approach for acquiring a foreign language, allowing for a large amount of language exposure. However, the influence of supplementary activities within ER programs has yet to be thoroughly investigated, despite previous studies suggesting their potential effectiveness. This research investigated the effects of ER combined with supplementary instructional approaches on vocabulary acquisition and reading strategies among university EFL students in Japan. Two supplementary instructional approaches were implemented within ER programs of equivalent duration, learner proficiency levels, and grade levels: one emphasizing reading volume and the other focusing on directing attention to unknown words while and after reading. The results revealed that directing learners' focus to unknown words led to more pronounced growth in vocabulary size. However, when attention was directed towards unfamiliar words, students were less inclined to infer their meanings from context and more likely to resort to using dictionaries. Conversely, placing emphasis on increasing reading volume increased the likelihood of students encountering engaging books and series, and reading materials at a specific readability level. These findings indicated that supplementary activities within ER programs influenced learning outcomes and reading strategies. Consequently, when integrating supplementary activities into the ER program, educators should carefully consider the program's objectives and incorporate suitable activities accordingly.