  In this work we fit the available binned data of the differential decay
distribution of the exclusive $B \to K^{(*)} \ell^+ \ell^-$ and $B_s \to \phi
\mu^+ \mu^-$ decays to the mass and mixing parameters of a light dark vector
boson model. Due to an incorrect assessment of the dominant contribution of the
dark $Z$ model to the FCNC B-meson decays, a previous work in literature
reported that $\mathcal{O}(1)$ mixings were allowed by the data. In this talk
we report the correct calculations and constraints on the mixing parameters as
well as the mass of the dark vector boson using the marginalization technique.
We also study other relevant bounds on the parameter space from low energy
experiments such as the atomic parity violation, $K^+ \to \mu^+ + invisible$
decay, $B_s - \overline{B}_s$ mixing etc and find that inspite of obtaining a
good fit to the experimental $b \to s \ell^+ \ell^-$ data, the entire parameter
space gets ruled out from some of the above bounds. In case of a model with
tiny mixing and additional interaction of the dark $Z$ to the muon, some bounds
are relaxed while some others are violated.
