<p>This paper presents a vector-based hypoplastic model for the interface behavior of granular materials. By applying a unit normal vector, the original tensorial model is reduced to in-plane projections of stress and strain rate onto the contact surface. To model the shear-band formation in granular soils, a strain-gradient extension is incorporated into the vector-based hypoplastic model. By introducing an inherent length scale, a finite shear-band thickness in the post-localization regime can be reproduced. The proposed model inherits the simplicity of the original hypoplastic model while enabling shear-band analysis under interface conditions. Numerical simulations show that the proposed model captures the salient features of granular soil-structure interface behavior. In addition, the onset of localization and shear-band thickness are governed by the gradient boundary condition imposed at the contact surface.</p>