  Hyper-Raman scattering has been measured on vitreous boron oxide,
$v-$B$_2$O$_3$. This spectroscopy, complemented with Raman scattering and
infrared absorption, reveals the full set of vibrations that can be observed
with light. A mode analysis is performed based on the local D$_{3h}$ symmetry
of BO$_3$ triangles and B$_3$O$_3$ boroxol rings. The results show that in
$v-$B$_2$O$_3$ the main spectral components can be succesfully assigned using
this relatively simple model. In particular, it can be shown that the
hyper-Raman boson peak arises from external modes that correspond mainly to
librational motions of rigid boroxol rings.
