VX-III is better than Vari-X III optically. Not by a huge margin, but better nonetheless. Vari-X III had "Multicoat 4" lens coatings, and VX-III has their newer coatings called "Index Matched." The current VX-II is equivalent to the old Vari-X III. I've compared the 2 series and to my eye, the VX-III appears to have a little better resolution and light transmission. I believe the VX-III has a couple other model options that wasn't offered in the Vari-X III line, but it's hard to keep track of all Leupy's model variations; there's just so many.
There's nothing wrong with the Vari-X IIIs. I have a couple that have given me good service and don't plan to replace them.