  Quantum electrodynamics predicts that the quantum vacuum is birefringent, but
due to the very small cross-section this is yet to be confirmed by experiment.
Vacuum birefringence arises as the elastic part of photon-photon scattering;
the inelastic part is Breit-Wheeler pair-production. We outline how
measurements of the photon-polarised nonlinear Breit-Wheeler process can be
used to infer a measurement of nonlinear vacuum birefringence. As an example
scenario, we calculate the accuracy of such a measurement for parameters
anticipated at upcoming laser-particle experiments.
