  We develop a unitarity approach to consider the final state interaction
corrections to the tree level graphs calculated from Chiral Perturbation Theory
($\chi PT$) allowing the inclusion of explicit resonance fields. The method is
discussed considering the coupled channel pion and kaon vector form factors.
These form factors are then matched with the one loop $\chi PT$ results. A very
good description of experimental data is accomplished for the vector form
factors and for the $\pi\pi$ P-wave phase shifts up to $\sqrt{s}\lesssim 1.2$
GeV, beyond which multiparticle states play a non negligible role. In
particular the low and resonance energy regions are discussed in detail and for
the former a comparison with one and two loop $\chi PT$ is made showing a
remarkable coincidence with the two loop $\chi PT$ results.
