Diffraction is a property of an aperture. The shape of that aperture impacts the diffraction. And because of that, mechanical vignetting impacts the shape of the aperture off axis at large apertures--small f-numbers. As hpulley also stated, aberrations can impact diffraction. And because nothing is simple, lenses can be optimized for resolution or contrast, but not both at the same time. This mix between the two will also affect diffraction.

But I concur with hpulley, he probably did not understand what he was saying.