Flash flood is the most recurrent natural threat in the northeastern part of India, especially during the peak of the monsoon season. Considering the exponential rise in the frequency of flash flood events, identifying the Flash Flood Vulnerable Zones (FFVZs) is one of the most crucial findings to limit their negative consequences. In this study, Grey Fuzzy Analytic Hierarchy Process (GFAHP) based model integrated with the Geographic Information System (GIS) is implemented to assess the flash flood vulnerability in the Ranikhola watershed, East Sikkim, India. The GFAHP combines the benefits of all the traditional Multi-Criteria Decision-Making (MCDM) approaches. The Fuzzy Analytic Hierarchy Process (FAHP) is used to compute the weights, while the weight coefficients are evaluated using the Grey method. A novel Flash Flood Vulnerability Index (FFVI) is proposed to assess the flash flood vulnerability that takes into account twelve natural and anthropogenic parameters. Further, the FFVI map is classified into three FFVZs: low, moderate, and high. The efficacy of the flash flood vulnerability map is calculated by a single effectiveness value (EV), and the result is between moderate and more significant. The sensitivity analysis results show the influence of individual attributes on FFVI and FFVZs.