Flooding represents the most persistent and devastating natural hazard in North Bihar, India, causing extensive socio-economic disruption annually and affecting millions of vulnerable populations. This study presents a comprehensive assessment of composite flood vulnerability across seven districts of the Ghaghara-Gandak zone. Employing the IPCC's tripartite vulnerability framework, we synthesized detailed exposure, sensitivity, and adaptive capacity indicators through FVI methodology and spatial analysis techniques. The research utilized multi-source datasets including census data, administrative records, and socio-economic indicators spanning 1990-2022. Results reveal pronounced spatial disparities in vulnerability patterns, with East Champaran exhibiting the highest vulnerability index (1.24), followed by West Champaran (0.90) and Muzaffarpur (0.76). These districts demonstrate critically high exposure levels combined with limited adaptive capacity, creating conditions for severe and recurring flood impacts. Conversely, Siwan shows the lowest vulnerability (0.24), benefiting from reduced exposure and enhanced adaptive capacity. The study contributes to vulnerability science by demonstrating the application of composite indexing in data-constrained environments while providing actionable insights for targeted disaster risk reduction and climate adaptation strategies in one of India's most flood-prone regions