<p>This study addresses vibration control of a gradually varying-size beam supported manipulator system (VBSMS) under on-orbit assembly conditions, achieved by incorporating a nonlinear energy sink (NES) consisting of a cubic nonlinear spring in parallel with a linear spring. A dynamic model of the coupled system is developed, and the frequency and amplitude responses are obtained using the harmonic balance-alternating frequency/time (HB-AFT) method in conjunction with the arc-length continuation technique. A composite optimization objective is formulated, which considers both the suppression ratio of the maximum steady-state response and a supplementary indicator that evaluates the overall performance of the absorber by capturing the severity of the vibration amplification and the extent of the affected region. A parametric analysis was subsequently conducted, leading to the proposal of optimal design guidelines for the NES parameters. Focusing on the response amplitude over a selected frequency range, the analysis yields optimal design criteria that minimize the steady-state amplitude of the beam throughout the assembly process, thereby offering practical guidance for absorber design in VBSMS.</p>