Abstract We show that the generating functional Z for non-abelian vector scattering amplitudes in the open superstring theory can be represented as a string path integral average of the 1D supersymmetric generalization of the Wilson tr P exp-factor. The amplitudes generated by Z are manifestly gauge invariant, i.e. they automatically contain contact contributions ignored in the standard expressions for the amplitudes. They are also free of super Möbius infinities and hence can be computed without fixing a super Möbius gauge. This implies that the renormalized value of Z (expanded in powers of momenta) coincides with the string theory effective action for the vector field.