Understanding how words are created is potentially a key component to being able to learn and understand new vocabulary words. However, research on morphological awareness is relatively rare. In this study, over 660 preschool-aged children from three language groups (Cantonese, Mandarin, and Korean speakers) in which compounding morphology is highly prevalent were tested on their abilities to manipulate familiar morphemes to create novel compound words as well as on a variety of early language and reasoning measures twice over the span of 9 months to 1 year. With Time 1 vocabulary knowledge, phonological processing, and reasoning skills controlled, morphological awareness predicted unique variance in Time 2 vocabulary knowledge across languages. Across languages, vocabulary knowledge also predicted unique variance in subsequent morphological awareness, with Time 1 morphological awareness controlled. Findings underscore the bidirectional bootstrapping of morphological awareness and vocabulary acquisition for languages in which lexical compounding is prominent, and suggest that morphological awareness may be practically important in predicting and fostering children's early vocabulary learning.