<p>Urban shrinkage has become a critical global concern, prompting growing calls to enhance urban vitality as a pathway toward sustainable development. Focusing on the Harbin–Changchun Urban Agglomeration (HCUA) in Northeast China, a region marked by widespread shrinkage, this study proposes a dual-scale analytical framework that bridges urban and regional levels to evaluate spatiotemporal vitality patterns and their influencing factors. At the urban scale, a multidimensional index system encompassing economic, social, environmental, and cultural dimensions is constructed to assess vitality levels, identify temporal dynamics, and diagnose systemic obstacles. The findings reveal a persistent ‘dual-core–satellite’ structure, highlighting both spatial concentration in core cities and peripheral disparities. At the regional scale, multi-source geospatial big data and a grid-based method are employed to capture the diurnal rhythms and seasonal fluctuations of urban vitality, revealing the interplay among human activities, the built environment, and spatial vitality. Crucially, the relationship between shrinkage and vitality is shown to be non-linear and heterogeneous. Some shrinking cities demonstrate dimension-specific resilience in cultural and environmental domains, challenging the dominant view that shrinkage entails inevitable decline. This study contributes an adaptable framework that transcends administrative boundaries, offering methodological and empirical insights into the evaluation and enhancement of urban vitality, while supporting context-sensitive governance strategies in shrinking urban regions.</p>