A two-state diabatic model for high-energy photoionization is proposed and applied to the inner valence x-ray photoelectron spectrum of acetylene. The diabatic electronic representation is accomplished by calculating the first order nonadiabatic coupling over the vibrational normal modes. The photoionization overlap amplitudes are expressed in terms of this new basis which is related to the adiabatic representation via an orthogonal transformation. The nonadiabatic (vibronic) coupling leads to a significant redistribution of photoelectron intensity for the totally symmetric states in the inner valence region. This finding resolves previous ambiguities in the analysis of the spectrum.