The optical-optical double-resonance multiphoton-ionization spectra through the C 2Π and D 2Σ+v'= 4 intermediate states display sharp autoionization peaks attributed to vibrationally excited ns and nd Redberg series converging to the v+ = 4 level of the NO+, 1Σ+ state. Photoelectron mesurements up to n = 14 level of these Rydberg series indicate that Rydberg-valence multistate interactions in the ionization continuum play an essential role in experimentally determined vibrational state distributions of NO+. The autoionization mechanism proposed here indicates that the ionization transition takes place via a dissociative valence excited state which electronically interacts with the autoionizing Rydberg state. The vibrational state populations thus induced are described solely by the Franck-Condon overlap between the vibrational wavefunctions of the repulsive potential associated with the valence excited state and the bound potential of the molecular ion.