Sloshing is one of the critical problems in aerospace vehicles with liquid containers. Motion of the liquid in resonance situations can degrade the stability and performance of attitude control systems. Two important characteristics of this time varying phenomenon are sensorless and underactuated properties which lead to difficulty of attitude control system design. In this paper, a new technique based on soft sensor and virtual actuator is used to suppress the effects of fuel sloshing in an aerospace launch vehicle (ALV). For this purpose, a nonlinear dynamic model of the vehicle with mechanical model of the fuel sloshing is considered as a multibody dynamic system. The preliminary attitude control system of the vehicle is extended using the new vibration suppression technique and a numerical simulation of the nonlinear model is carried out. Results of the simulation show that the undesired effects of the fuel sloshing are effectively decreased using the proposed vibration suppression technique.