A hypothesis is presented in this paper for evaluating the root mean square (RMS) errors in acceleration for determining the optimal locations of the exciters during the vibration testing of slender flexible structures, e.g., missiles, rockets, and space vehicles. Simulation studies are carried out on a realistic slender nonuniform beam in a free–free conditions to characterize the error estimations in the desired and achieved acceleration spectra at different locations of the structure. The optimal locations of the exciters are obtained using a real-coded genetic algorithm (RCGA) with minimal required force levels and the corresponding RMS error estimates. It has been observed that RMS force levels are a minimum when the exciter and control sensor locations are noncollocated. This study also provides a methodology in deriving the optimum force levels required for the environmental vibration testing of the full vehicle structure. The RMS error levels are found to be a minimum at the control sensor locations.