A simple model of harmonic vibrations in topologically disordered systems, such as glasses and supercooled liquids, is studied analytically by extending Euclidean random matrix theory to include vector vibrations. Rather generally, it is found that (i) the dynamic structure factor shows soundlike Brillouin peaks whose longitudinal/transverse character can only be distinguished for small transferred momentum, p; (ii) the model presents a mechanical instability transition at small densities, for which scaling laws are analytically predicted and confirmed numerically; (iii) the Brillouin peaks persist deep into the unstable phase, the phase transition being noticeable mostly in their linewidth; (iv) the Brillouin linewidth scales like p(2) in the stable phase, and like p in the unstable one. The analytical results are checked numerically for a simple potential. The main features of glassy vibrations previously deduced from scalar are not substantially altered by these new results. (C) 2003 American Institute of Physics.