  For scattering amplitudes in strong background fields, it is -- at least in
principle -- possible to perturbatively expand the background to obtain
higher-point vacuum amplitudes. In the case of self-dual plane wave backgrounds
we consider this expansion for two-point, one-loop amplitudes in pure
Yang-Mills, QED and QCD. This enables us to obtain multicollinear limits of
1-loop vacuum amplitudes; the resulting helicity configurations are
surprisingly restricted, with only the all-plus helicity amplitude surviving.
These results are shown to be consistent with well-known vacuum amplitudes. We
also show that for both abelian and non-abelian supersymmetric gauge theories,
there is no helicity flip (and hence no vacuum birefringence) on any plane wave
background, generalising a result previously known in the Euler-Heisenberg
limit of super-QED.
