  We study the thermodynamic properties of a neutral vector boson gas in
presence of a constant magnetic field, by means of a semi-classical approach
that allows to introduce the spin in the non-relativistic spectrum of the
bosons. Bose-Einstein condensation is obtained and it turns out to depend on
all the parameters involved in the problem: temperature, particle density and
magnetic field. An spontaneous magnetization appears at low temperature as a
consequence of the condensed state. The axial symmetry imposed in the system by
the magnetic field presence, splits the pressure in two components, one along
and another perpendicular to the magnetic axis. Under certain conditions, the
perpendicular pressure becomes negative signaling that the system undergoes a
transversal magnetic collapse. The spontaneous magnetization might be useful to
model magnetic field production inside compact stars, while the negative
pressures imposes certain limits to the temperatures and densities needed
inside these objects to support a given magnetic field.
