Light vectors coupled to bosonic currents

New spin-1 particles with masses below the weak scale are present in many theories of beyond Standard Model (SM) physics. In this work, we extend previous analyses by systematically considering the couplings of such a vector to the bosonic sector of the SM, focusing on models that lead to mass mixing with the Z boson. These couplings generically lead to enhanced emission of the vector's longitudinal mode, both in Higgs decays and in flavor changing meson decays. We present bounds in the SM+X effective theory and investigate their model dependence. For the case of Higgs decays, we point out that tree-level vector emission is, depending on the model, not always enhanced, affecting the constraints. For meson decays, which are the dominant constraints at small vector masses, we find that while B decay constraints can be weakened by fine-tuning UV parameters, it is generically difficult to suppress the stringent constraints from kaon decays.