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Voluntary social insurance (VSI) is critical to extending social protection coverage to informal sector workers in developing countries like Vietnam, yet participation rates remain low. This study investigates the behavioral, social, and structural determinants influencing VSI participation, integrating the theory of planned behavior (TPB) and rational choice theory (RCT) into a comprehensive framework. Data were collected through a structured survey of 448 informal workers and analyzed using structural equation modeling (SEM). Results demonstrate that positive attitudes, supportive subjective norms, respected community figures, and community cohesion drive VSI participation, whereas financial incentives and flexible payment options alone do not. The study highlights the dominant role of internal motivations and social trust over purely economic considerations. Practical implications emphasize the need for communication strategies fostering positive perceptions, leveraging community leadership, and reinforcing institutional credibility. These findings provide valuable insights for policymakers aiming to design more inclusive, resilient social protection systems tailored to transitional economies like Vietnam. Plain Language Summary • Internal motivations and social trust outweigh financial incentives in driving participation in Vietnam’s voluntary social insurance (VSI). • Positive attitudes, supportive norms, and respected community figures significantly influence informal workers’ decision to enroll in VSI. • Flexible payment schemes and government subsidies alone are insufficient to boost VSI enrollment without addressing trust and awareness. • Policy strategies must prioritize trust-building, community engagement, and reshaping public perceptions to expand social protection coverage.