Background: The promotion of breastmilk substitutes (BMS) is an important barrier to successful breastfeeding. Objective: To examine the enactment and implementation of the Code of Marketing of Breast-Milk Substitutes (the Code) in Vietnam with a focus on marketing practices by the baby food industry and perceptions of caregivers, health workers, and policy makers. Methods: From May to July 2020, we conducted a mixed-method, cross-sectional study including a survey of 268 pregnant women and 726 mothers of infants aged 0–11 months and in-depth interviews with a subset of interviewed women (n = 39), policy makers, media executives, and health workers (n = 31). Results: In the previous 30 days, two mothers (out of 726) participating in the quantitative survey reported that health workers had recommended BMS, at private hospitals in both cases. In-depth interviews with health workers showed that hospitals have internal procedures to prevent the promotion of BMS by health workers. However, companies employed representatives to promote products not covered under the Code (e.g., commercial milk formula for pregnant women) at ante-natal care visits and by gaining contact information from women and using this information to promote BMS outside the hospital, often on social media. In the 30 days preceding the survey, one-fifth of pregnant women were exposed to promotions of commercial milk formula for pregnant women and 7.1% to promotions of BMS. Among mothers of infants, 7.3% and 10.7% of respondents with infants aged 0–5 and 6–11 months, respectively, were exposed to some form of BMS promotion in the past 30 days. Around the time of birth, parents commonly brought BMS to maternity facilities (52.5%) or purchased it nearby (35.4%). Conclusions: Although Vietnam has a strong regulatory environment for the protection, promotion, and support of breastfeeding, there are implementation, monitoring, and enforcement gaps. Stronger enforcement of national policies to regulate the pres-ence of BMS industry representatives in health facilities—both public and private—and the promotion of BMS products on digital platforms are needed.