Cz-Si samples, initially subjected to thermal treatments under high hydrostatic pressure, were subsequently irradiated by fast neutrons. This paper describes a series of infrared spectroscopy measurements that enabled us to determine the effect of the pre-treatments on the annealing characteristics of the VO defect in Si. We found that the activation energies of the two main annealing reactions: VO + O i → VO 2 and VO + Si I → O i that the defect participates, are comparatively smaller than those of initially untreated samples, correspondingly. We argue that the pre-treatments reduce the potential barrier for the migration of the VO defect (VO + O i → VO 2) and also reduces the binding energy of the Si I's, bound at large defect clusters (VO + Si I → O i).