This study examines how a policy mix shapes enterprise development in Vietnam’s open economy. Using cross-sectional survey data from 167 firms, the paper tests the joint and individual effects of seven policy domains: trade policy, enterprise support, taxation, credit access, infrastructure, market information, and the legal environment. The empirical results show that all seven policy dimensions are positively and significantly associated with enterprise development, with trade policy exerting the strongest effect, followed by enterprise support. The model explains 61.2% of the variance in enterprise development, indicating substantial explanatory power. The study contributes to the literature by showing that firm development is driven not by a single policy lever, but by policy complementarity within a coordinated institutional environment. By moving beyond narrow, single-policy explanations, the paper offers a more integrated account of how trade openness and domestic policy conditions jointly shape firm-level outcomes in an emerging economy.