I have studied that the hadronic matrix element of pion decay (Pi+->mu+ anti muon neutrino) is given as <0|ubar gamma[mu](1-gamma)d|pion>. The vector current does not seem to contribute, because it cannot connect a state of unnatural parity to hadronic vacuum. Only the axial and pseudoscalar current seems to conribute. Similarly for pion decay (Pi+>Pi0 mu+ anti muon neutrino) is given as <0|ubar gamma[mu](1-gamma)d|pion>. Now The vector current does contribute. The axial current contribution seems to vanish now because a state of unnatural parity is going to unnatural parity! I do not understand this abstract kind of reasoning.