  Due to a recent more precise evaluation of $V_{ud}$ and $V_{us}$, the
unitarity condition of the first row in the Cabibbo-Kobayashi-Maskawa (CKM)
matrix: $|V_{ud}|^2 + |V_{us}|^2 + |V_{ub}|^2 = 0.99798 \pm 0.00038$ now stands
at a deviation more than $4\sigma$ from unity. Furthermore, a mild excess in
the overall Higgs signal strength appears at about $2\sigma$ above the standard
model (SM) prediction, as well as the long-lasting discrepancy in the
forward-backward asymmetry ${\cal A}_{\rm FB}^b$ in $Z\to b\bar b$ at LEP.
Motivated from the above three anomalies we investigate an extension of the SM
with vector-like quarks (VLQs) associated with the down-quark sector, with the
goal of alleviating the tension among these datasets. We perform global fits of
the model under the constraints coming from the unitarity condition of the
first row of the CKM matrix, the $Z$-pole observables ${\cal A}_{\rm FB}^b$,
$R_b$ and $\Gamma_{\rm had}$, Electro-Weak precision observables $\Delta S$ and
$\Delta T$, $B$-meson observables $B_d^0$-$\overline{B}_d^0$ mixing, $B^+ \to
\pi^+ \ell^+ \ell^-$ and $B^0 \to \mu^+ \mu^-$, and direct searches for VLQs at
the Large Hadron Collider (LHC). Our results suggest that adding VLQs to the SM
provides better agreement than the SM.
