I emailed Leupold and this was the response I got.

"The VX-3i is actually incrementally better than the VX-3 (VX-3i stands for VX-3 improved). We spent a lot of money at the end of 2015 bringing the VX-3 assembly line into the 21st century with the purchase of new machinery. We gained manufacturing efficiencies which have been passed onto the consumer."