I've had both, still have the "i" version. Optically I couldn't tell a difference between the vx3 and vx3i. Aesthetically the "i" version has a shorter power ring adjuster with a more ergonomic ring, it's a hair shorter overall and I find it better looking to my eye(s). They're pretty much the same scope but obviously you can only go to the store and buy the latest version. The last one I got at wholesale was $589 with a $75 instant rebate.
This is the part of the scope that is different between the two models but I haven't got a picture of the older vx3, easy to find one online though.