Virtual laboratories have become an increasingly important component of science education by providing flexible and technology-enhanced environments for experimental learning. Despite growing evidence of their effectiveness, the pedagogical mechanisms through which they improve learning remain insufficiently understood. This study addresses this gap through an Integrative Cross-National Evidence Synthesis of nine empirical studies, systematic reviews, and meta-analyses conducted across diverse educational contexts and STEM disciplines. The findings indicate that virtual laboratories consistently enhance conceptual understanding, academic achievement, self-efficacy, confidence, and laboratory readiness, while their influence on learning motivation is more dependent on instructional design and contextual factors. Four complementary pedagogical mechanisms emerged from the synthesis: cognitive scaffolding, psychological safety, scientific visualization, and hybrid learning integration. The evidence further demonstrates that virtual laboratories achieve the greatest educational value when integrated with physical laboratories rather than used as standalone replacements. A mechanism-based conceptual framework is proposed to explain how virtual laboratories support meaningful learning and to inform educators, curriculum designers, and policymakers in developing effective and sustainable hybrid laboratory ecosystems for science education.