I was wondering the same thing between the scopes so did some research here is what I was able to find out the difference's between the two lines were.
The VX-6HD and VX-5HD both have the same glass and coatings. The difference you will see is in the features of the scopes.
1) The VX-6HD has a 6:1 magnification ratio, the VX-5HD has a 5:1 magnification ratio
2) All VX-6HD scopes come with 2 turn zero locking CDS dials for elevation and zero lock dials for windage, the VX-5HD you are looking at comes with 2 turn zero locking CDS dials for elevation but comes with capped pop up re-zero dials for windage
3) The VX-6HD line has electronic in-scope cant indicator, the VX-5HD does not have this feature
4) The VX-6HD comes with the alumina flip back lens covers, the VX-5HD comes with the bungee style covers
5) The VX-6HD comes with a magnification selector throw lever, the VX-5HD is available from Leupold but it is an aftermarket purchase
Those are the 5 difference's between the two line ups. I've got the the Impact 29 in the VX-3I LRP scopes I use them for varmint hunting/long range fun and they have worked great for me. I am still deciding between the VX-6HD and the VX-5HD the biggest factor for me is the 2 turn elevation dials.