  Recent high precision determinations of $V_{us}$ and $V_{ud}$ indicate
towards anomalies in the first row of the CKM matrix. Namely, determination of
$V_{ud}$ from superallowed beta decays and of $V_{us}$ from kaon decays imply a
violation of first row unitarity at about $4\sigma$ level. Moreover, there is
tension between determinations of $V_{us}$ obtained from leptonic $K\mu2$ and
semileptonic $K\ell3$ kaon decays. These discrepancies can be explained if
there exist extra vector-like quarks at the TeV scale, which have large enough
mixings with the lighter quarks. In particular, extra vector-like weak singlets
quarks can be thought as a solution to the CKM unitarity problem and an extra
vector-like weak doublet can in principle resolve all tensions. The
implications of this kind of mixings are examined against the flavour changing
phenomena and SM precision tests. We consider separately the effects of an
extra down-type isosinglet, up-type isosinglet and an isodoublet containing
extra quarks of both up and down type, and determine available parameter spaces
for each case. We find that the experimental constraints on flavor changing
phenomena become more stringent with larger masses, so that the extra species
should have masses no more than few TeV. Moreover, only one type of extra
multiplet cannot entirely explain all the discrepancies, and some their
combination is required. We show that these scenarios are testable with future
experiments. Namely, if extra vector-like quarks are responsible for CKM
anomalies, then at least one of them should be found at scale of few TeV, and
anomalous weak isospin violating $Z$-boson couplings with light quarks should
be detected if the experimental precision on $Z$ hadronic decay rate is
improved by a factor of $2$ or so.
