  We reconsider the impact of heavy vector-like fermions on the couplings of
standard model (SM) quarks to the SM gauge bosons W and Z and to the SM Higgs
boson H. Integrating out these fermions at tree level we derive general
formulae that can be used in any model containing such particles. We apply
these formulae to the case of the lightest Kaluza-Klein (KK) fermions in a
Randall-Sundrum (RS) model with a custodial protection of flavour conserving
$Zd_L^i d_L^i$ and flavour violating $Zd_L^i d_L^j$ couplings. We point out
that in this model also the couplings of $Zu_R^i u_R^i$ and $Zu_R^i u_R^j$ are
protected. In particular we demonstrate explicitly that this protection is not
spoiled by the mixing of the SM quarks with the KK fermions, which is
guaranteed by the underlying P_LR symmetry. We find that the impact of KK
fermions on the Z-couplings, while not negligible, is significantly smaller
than the one coming from the mixing of Z with the heavy KK gauge bosons Z_H and
Z'.The situation is similar for the W couplings with the only exception of the
tb coupling where the impact of KK fermions can in principle be larger than the
effects that are induced by gauge boson mixing. We also show explicitly that at
O(v^2/M_{KK}^2) the fermion-Higgs couplings of a Higgs placed on the infrared
(IR) brane is not affected by the KK contributions up to strongly chirally
suppressed contributions. The corrections to the CKM matrix, in particular the
breakdown of its unitarity, are found to be small. We also investigate the
right-handed couplings of W that are generated through the mixing with
KK-fermions and introduce a 3x3 matrix that describes the pattern of flavour
violation in these new interactions of the standard W.
