Aim: To explore Vietnamese–Australian children’s proficiency and use of Vietnamese and English and identify associated factors that are related to demographics, language practices, language ideologies, and language management. Methodology: Vietnamese–Australian parents (n = 151) completed a questionnaire (in English or Vietnamese) regarding their child’s language proficiency and use, demographic details and a range of factors as conceptualized by Spolsky’s language policy theory: language practices; language ideologies; and language management. Data and analysis: Bivariate analyses (Pearson’s correlation and analysis of variance) and multiple regression models were conducted to explore associations between language proficiency and use and associated factors and identify the most significant factors. Findings/conclusions: Factors associated with children’s Vietnamese language proficiency (oral/written) included: demographic factors; language practices; language ideologies; and language management. In contrast, children’s English language proficiency (oral/written) was linked to demographic factors and language practices. Children’s Vietnamese language use was not significantly correlated with demographics but rather with language practices, language ideologies, and language management. Children’s home language use and proficiency did not have a negative impact upon their English proficiency. Originality: This study is the first to consider factors associated with Vietnamese–Australian children’s language proficiency and use. Significance/implications: Demographic factors, language practices, language ideologies, and language management were associated with children’s language proficiency and use. The results can be used by parents, educators, policy-makers, speech–language pathologists and other professionals to support Vietnamese–Australian and multilingual children around the world to develop and maintain their home and majority languages.