  We put forward a novel method to generate high-brilliance polarized
$\gamma$-photon beams via vacuum dichroism (VD)-assisted vacuum birefringence
(VB) effect. We split a linearly polarized (LP) laser pulse into two subpulses
with the first one colliding with a dense unpolarized electron beam to generate
LP $\gamma$ photons (via nonlinear Compton scattering), which then further
collide with the second subpulse and are partially transformed into circularly
polarized ones via the VB effect. We find that by manipulating the relative
polarization of two subpulses, one can ``purify'' (i.e., enhance) the
polarization of the $\gamma$-photon beam via the VD effect. Due to the VD
assistance, the VB effect reaches optimal when the relative polarization is
nearly $30^\circ$, not the widely used $45^\circ$ in the common VB detection
methods. In addition, our method can be used to efficiently confirm the
well-known VB effect itself, which has not been directly observed in
experiments yet.
