Vegetation plays a critical role in regulating the catchment water balance and enhancing soil stability through root reinforcement. The dynamic nature of vegetation, particularly its seasonal change, significantly affects the magnitude of this influence. However, quantifying the long-term impacts of dynamic vegetation on both flood and landslide occurrences at the catchment scale remains challenging due to the complexity of root structures and the varying dimensions of landslides. In this study, we improved the coupled hydrological-geotechnical model iHydroSlide3D v1.0 by incorporating key vegetation components, such as Leaf Area Index (LAI), root characteristics, and their seasonal dynamics. The improved model was validated using historical observations and applied to a 100-years simulation driven by a weather generator. Three computational scenarios were employed to assess the influence of vegetation on key hydrological and slope-stability variables. Results show that vegetation reduces soil moisture and runoff during low to moderate rainfall events but has a limited impact during larger rainfall events. Additionally, slope stability is found to be more influenced by root reinforcement than soil water uptake. The dynamic nature of vegetation plays a decisive role in modulating its effects on hydrological processes and soil stability, depending on the growth or decay trend of vegetation. This modeling framework offers a robust tool for assessing long-term flood and landslide risks in vegetated catchments.