Void formation and swelling were studied in nickel-ion-bombarded vanadium, both with and without the presence of helium. Helium has a significant effect on void nucleation when the defect generation rate is low. At high defect generation rates, helium has little effect on void nucleation and the resultant swelling. The data are in reasonably good agreement with the nucleation theory of Wiedersich and Katz based on void nucleation on single gas atoms. The results demonstrate the importance of helium and defect generation rate in ion simulation experiments.