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New dark vector bosons that couple very feebly to regular matter can be produced in the early universe and decay after the onset of Big Bang Nucleosynthesis (BBN) or the formation of the cosmic microwave background (CMB) at recombination. The energy injected by such decays can modify the light element abundances or modify the power and frequency spectra of the CMB. In this work we study the constraints implied by these considerations on a range of sub-GeV dark vectors including the kinetically mixed dark photon, as well as gauge B-L and lepton families. We focus on the effects of electromagnetic energy injection, and we update previous investigations of the dark photon by taking into account non-universality in the photon cascade spectrum relevant for BBN and the energy dependence of the ionization eciency after recombination in our treatment of modications to the CMB.