Notwithstanding its defining feature of normative pluralism, the socialist state of Vietnam basically adopts a legal centralist approach to regulation. The judiciary is arguably the most illustrative of this approach, since it is the main forum where legal centralism encounters normative pluralism. Our research examines the choice of norms in judicial adjudication in Vietnam to check the effectiveness of its legal centralist approach. It finds that, despite lacking institutional support, judges managed to apply customary norms at their discretion against the state’s emphasis on top-down legal rules. A legitimacy-based analysis explains this phenomenon. It points out that judges conceptualized their legitimacy under the influence of both legal and extra-legal rules, thus making it apart from the legality. Judges attempt to bridge the gap between legitimacy and legality enabled de factor normative pluralism. In looking at the influence of customary norms over judicial adjudication, the article aims to make both theoretical and practical contributions. Theoretically, it enriches the scholarship of normative pluralism by showing how legitimacy-building keeps normative pluralism effective, irrespective of the dominating legal centralism. Practically, it proffers insightful implications for the ongoing court reforms in Vietnam based upon the findings.