I believe the 4x24 have the 34mm tube and all 3x18's have a 30mm tub, but not 100% sure.
When you get the VX6, let me know if you can turn the magnification dial.
I have two 2x12's and they both seem to have more friction than I would like, and one has uneven friction...i.e. at 6x or so it gets harder to turn, then let's up. I live 10 minutes from leupold so I may take them in to see if that is normal at all.