  We examine the vector-portal inelastic dark matter (DM) model with DM mass
$m_\chi$ and dark photon mass $m_{A'}$, in the `forbidden dark matter' regime
where $1 \lesssim m_{A'}/m_\chi \lesssim 2$, carefully tracking the dark sector
temperature throughout freezeout. The inelastic nature of the dark sector
relaxes the stringent cosmic microwave background (CMB) and self-interaction
constraints compared to symmetric DM models. We determine the CMB limits on
both annihilations involving excited states and annihilation into $e^+e^-$
through initial-state-radiation of an $A'$, as well as limits on the DM
self-scattering, which proceeds at the one-loop level. The unconstrained
parameter space serves as an ideal target for accelerator $A'$ searches, and
provides a DM self-interaction cross section that is large enough to observably
impact small-scale structure.
