In the context of the “dual carbon” goal and high-quality development, promoting urban vitality enhancement and carbon reduction has become a critical issue for sustainable urban development. Taking Fuzhou, a typical mountain-sea city, as an example, a six-dimensional urban vitality evaluation system encompassing social, economic, cultural, spatial, environmental, and policy dimensions was constructed based on multi-source data. Combined with carbon emission data and using townships as the basic analytical unit, bivariate spatial autocorrelation and OPGD (optimal parameters-based geographical detector) were employed to reveal the spatial differentiation and driving pathways of urban vitality and carbon emissions. The findings reveal that Fuzhou exhibits a concentric circle structure characterized by “high vitality with high carbon emission agglomeration in the center, and low vitality with low carbon emissions in the periphery.” However, policy-driven new urban areas, such as the Changle Coastal New Town, display a “leapfrog” high-value zone where carbon emissions outpace vitality, deviating from the traditional concentric pattern. Social, economic, cultural, spatial, and comprehensive vitality are significantly positively correlated with carbon emissions, whereas environmental and policy vitality show significant negative correlations, indicating that ecosystems and policy interventions play key regulatory roles in carbon reduction. Weibo check-in density, land-use functional mix, and GDP are identified as the core explanatory factors for the spatial heterogeneity of carbon emissions, with their interactions exhibiting nonlinear synergistic amplification effects. Additionally, the total fishery production value and TPI (topographic position index) exert significant driving forces on carbon emissions, highlighting the unique characteristics of mountain-sea cities. The study proposes strategies including precise zonal regulation, coordinated policy-environment governance, and the construction of a resilient mountain-sea synergistic pattern, providing scientific evidence and decision-making references for achieving synergistic enhancements in vitality and carbon reduction in mountain-sea cities.