Key: same wide angle, not same lens.

Of course, if you only use the center part of a lens, it is better than the corner part. But I am talking about the same angular size of a photo: For a cropped camera, you will need a different lens (shorter focal length). Now you have two choices:

1. Get a full frame lens with 1/1.6 x the focal length of the original lens, and use it on the crop camera to achieve the original ultra wide angle (NOW, YOU ARE TALKING ABOUT ULTRA-ULTRA WIDE ANGLE). But then, why not using this lens on the 5D?

2. Get a crop lens, which will have its own vignetting at the corners.

So you see, crop camera will not help you with vignetting.