Liquid sloshing may be employed for vibration control of resonant structures, similar to that of a classical tuned vibration absorber. For such a case, the sloshing frequency is tuned at a critical frequency of the structure in order to gain the benefits of the pressure forces as control forces. Such an absorber is practically free of maintenance. The work presented in this paper utilizes a flexible container partially filled with water, as the sloshing absorber. Numerical predictions are presented where a “tuned” flexible container can be advantageous over a rigid container for effective control.