I was wondering, is it be possible to attach a Century Optics Achromatic Diopters to the end of Century Optics .7X Wide Angle Converter? Assuming it is possible, would there be any visual problems with such a setup (vignetting perhaps)?

I don't think it'll work. The diopters are 72mm thread mount, and the outer dimension of the .7x is something like 95mm or 105mm...