This study investigated contributors of English reading comprehension outcomes among predominantly U.S.-born 1st- and 3rd-grade (N = 73) dual language learners (DLLs) from Spanish-speaking, low-income homes who attend English-only instructional schools in the Southern region of the United States, which is experiencing historic rates of school-age DLL enrollment. We investigated the utility of various conceptualizations of vocabulary, namely English-only, Spanish-only, and specifically Spanish-English conceptually scored receptive vocabulary, in understanding DLLs' reading comprehension. We first examined whether a gap was evident between the various conceptualizations of vocabulary and English word reading. Then, using structural equation modeling, we investigated the influence of the various conceptualizations of vocabulary on English reading comprehension, accounting for English word reading skills. Finally, we examined the potential contributions of DLLs' home language environments. Results revealed that the gap between English word reading and vocabulary varied as a function of the conceptualization of vocabulary. Further, English word reading emerged as the robust contributor to children's English reading comprehension, with no significant influence of receptive vocabulary, regardless of how it was conceptualized. Finally, and contributing to a nascent area of research, attention to DLLs' home language use practices suggests that the productive language domain (i.e., children's own home language use) may represent an important contributor to English reading comprehension among DLLs from Spanish-speaking homes. We discuss theoretical and practical implications.