We discuss asymptotic freedom as an aspect of the response of a charged vector field to an external homogeneous magnetic field. It is shown that there exists an unstable mode of the field. The vacuum polarization is investigated. For lar is a "phase transition": the result of applying an external field and switching this field off is not equivalent to having no field at all. The vacuum polarization of the unstable mode corresponds to the self energy of a tachyon in 1 + 1 dimension.