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.
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