A multi-stakeholder spherical fuzzy MCDM framework for sustainable product development: Evidence from circular startups in Vietnam

The transition toward a circular economy has intensified interest in sustainable product development (SPD), particularly among startups operating under resource constraints and institutional uncertainty. While circular startups are increasingly recognized as critical agents of sustainable innovation, limited empirical research has systematically examined the multidimensional causal factors shaping their ability to develop sustainable products in emerging economies. Addressing this gap, this study proposes an integrated multi-criteria decision-making (MCDM) framework based on Spherical Fuzzy Sets (SFS) combined with the Delphi and DEMATEL methods to analyze and prioritize the determinants of SPD in circular startups. Drawing on 35 expert judgments from policymakers, investors, entrepreneurs, academics, and sustainability practitioners in Vietnam, the framework applies SFS-Delphi screening to retain 23 critical factors from an initial pool of 30, which are subsequently analyzed using SFS-DEMATEL to uncover causal relationships and structural hierarchies. Unlike conventional fuzzy-MCDM approaches that yield static factor rankings, the SFS-DEMATEL framework simultaneously captures expert hesitation and causal directionality, enabling a more nuanced identification of strategic leverage points in sustainability-oriented startup ecosystems. The results reveal that entrepreneurial commitment, R&D investment, stakeholder alignment, and environmental compliance act as dominant causal drivers shaping sustainable product outcomes, whereas several commonly emphasized policy instruments exert only indirect or limited influence in the Vietnamese context. This study contributes to the open-innovation literature by advancing a context-sensitive, uncertainty-aware causal decision framework for sustainability-oriented entrepreneurship. Practically, it provides actionable insights for startup founders, investors, and policymakers seeking to strengthen circular innovation ecosystems in emerging markets.