I did purchase a 5" vise to replace my aging 4" vise on my X3. It fits OK but it does overhang to the point that it's difficult to read the y axis handwheel. It adds a good bit of rigidity though so I use it for the big milling jobs (where handwheel readings often aren't an issue). But, as they say, you can't have too many vises. I did go back and buy a 4" vise which seems the best fit and if only one vise is to be had, that's the one I'd pick.
Cheers,
Phil