This paper outlines the vibration reduction of a spacecraft with flexible appendage subject to external disturbances. The hybrid dynamic model of the spacecraft is described by both partial differential equations and ordinary differential equations. A vibration control scheme consisting of two boundary control laws and a distributed control law is developed to restrain the spacecraft's vibration and track the desired attitude. An infinite-dimensional disturbance observer is introduced as a feedforward compensator to handle unknown external disturbances. Based on the semigroup theory, the extended LaSalle's invariance principle and Lyapunov analysis, the well-posedness and stabilities of the designed control system are both discussed. Simulation results are given for validating the effectiveness of the developed vibration control scheme.