Multilingualism provides cultural, economic and social benefits to individuals and societies. Many people with Vietnamese heritage have migrated to English-speaking countries such as Australia, Canada and the US. This study describes language proficiency, use and maintenance of 271 adults with Vietnamese heritage living across Australia. The majority were first-generation immigrants (76.6%), spoke Vietnamese as their first language (94.3%), and indicated Vietnamese was their most proficient language (78.5%). The majority were more likely to use Vietnamese (than English) with their mother, father, older siblings, Vietnamese-speaking grandparents, relatives in Vietnam, and Vietnamese friends. They used English and Vietnamese with their partners, children, younger siblings and English-speaking grandparents. They were more likely to speak English when working, studying and watching TV, but used English and Vietnamese equally on social media. The most important reasons for maintaining Vietnamese were: maintaining bonds with relatives, maintaining Vietnamese cultural identity, and building friendships.