Czochralski-grown Si samples were irradiated by fast neutrons, at room temperature, with the aim of studying the identity of the defects produced, using infrared spectroscopy. Two localized vibrational modes at 914 and 1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ were considered as intermediate defect stages between VO and ${\mathrm{VO}}_{2}$ complexes. We express the view that they may arise from a [VO+${\mathrm{O}}_{\mathit{i}}$] defect where an interstitial oxygen atom is trapped near a VO pair. Another two peaks at 1032 and 1043 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ were attributed to the ${\mathrm{VO}}_{4}$ defect and a tentative model of its structure is presented.