Leupold has a great way to know the quality of the scope. The facts are the more lens are the better the scope, the more light it gathers and the higher quality of scopes. So a VX I has one lens, a VX II has 2 and so forth up to VX 7 wich is the Caddy of all scopes. It has 7 lenses and gathers all color of light.
I'm not so sure it has as much to do with the number of lens as it does with the type of coating on the lens. As far as I know, they all have the same number of lens just the quality of lens and multicoat treatments are different though I could be wrong. I have never taken my Leupolds apart but I'm pretty sure the VX-I has more than one lens.