The 15-18 in lbs is straight from the horses mouth. That specification was given to me directly by Vortex. As I mentioned in another post, I set mine at 15 in/lbs with a "removable" grade loc-tite, you do not want a permanent bond here. While not necessary, the loc-tite will go a long way to ensure that those screws don't loosen up over time since you will most likely be using a lower than spec torque setting on your rings.
I'm also using a lapped set of Burris XTR rings, which have a very large contact surface compared to most "4 screw" type rings, so I'm sure this helps prevent any scope movement once torqued. Again the lapping, I feel is critical, as the closer to 100% contact you get, the more friction the rings have on the scope body, which obviously decreases the opportunity for any movement.
Again anyone with a vortex who has issues with POI shifting regularly, this is simply an "Over-Torque" issue in more cases than not. If the erector is bound even slightly, and doesn't move freely with turret adjustments, the hit of recoil can release any tension built up causing all of your issues.