  Using the 1/N_C expansion scheme and truncating the hadronic spectrum to the
lowest-lying resonances, we match a meromorphic approximation to the < S P P >
Green function onto QCD by imposing the correct large-momentum falloff, both
off-shell and on the relevant hadron mass shells. In this way we determine a
number of chiral low-energy constants of O(p^6), in particular the ones
governing SU(3) breaking in the K_{l3} vector form factor at zero momentum
transfer. The main result of our matching procedure is that the known loop
contributions largely dominate the corrections of O(p^6) to f_{+}(0). We
discuss the implications of our final value f_{+}^{K^0 \pi^-}(0)=0.984 \pm
0.012 for the extraction of V_{us} from K_{l3} decays.
