We determine the fraction f of B atoms belonging to boroxol rings in vitreous boron oxide through a first-principles analysis. After generating a model structure of vitreous B2O3 by first-principles molecular dynamics, we address a large set of properties, including the neutron structure factor, the neutron density of vibrational states, the infrared spectra, the Raman spectra, and the B-11 NMR spectra, and find overall good agreement with corresponding experimental data. From the analysis of Raman and B-11 NMR spectra, we yield consistently for both probes a fraction f of similar to 0.75. This result indicates that the structure of vitreous boron oxide is largely dominated by boroxol rings.