  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.
