As best I can understand it, the short simple answer is that the VX-I corresponds optically to the old Vari-XII, the VX-II corresponds to the Vari-XIII and that both the VX-III and the VX-3 are better yet. Apparently the VX-3 has worthwhile improvements over the VX-III. There are other differences also.

Details can be found here:

http://www2.leupold.com/resources/MyInfo81/Answerbook/findanswers.aspx

'Four legs good, two legs baaaad."
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"Jimmy, some of it's magic,
Some of it's tragic,
But I had a good life all the way."
(Jimmy Buffett)

SotG