  We consider the dependence of the pion and kaon decay constants on the up,
down and strange quark masses in QCD with strict isospin symmetry. The role of
dynamical vector meson degrees of freedom is scrutinized in terms of an
effective chiral Lagrangian for vector mesons. Applying a set of low-energy
parameters as determined previously from QCD lattice data on the masses of the
light vector mesons from PACS-CS, QCDSF-UKQCD and HSC we compute its
implications on the pion and kaon decay constants for QCD lattice ensembles of
HPQCD, CLS and ETMC. It is shown that with Gasser-Leutwyler L_4 and L_5
parameters fixed to the empirical decay constants an accurate reproduction of
their values at unphysical quark masses as computed by HPQCD, CLS and ETMC is
achieved. Results for the masses of the light vector meson, the \omega -\phi
mixing angles and the quark mass ratios for the ensembles used by HPQCD, CLS
and ETMC are discussed.
