I little behind in the thread but I'd like to do an update to what I've found. I did low light non scientific tests on a few scopes and this is what my eyes have been able to tell. Here are the scopes. VX3 4.5-14x50, VX3i 4.5-14x50, VX5HD 3-15x56 firedot, NF SHV 5-20x56, Swarovski z6i 2.5-15x56, Vortex VIper PST 4-16x50. Now here is how I rank them simply, based on low light visibility and clarity alone. Vortex-VX3-VX3i-SHV-(vx5,Z6). To be a little more descriptive. The VX3 scopes are equal to my eyes and are a fair amount brighter then the vortex. The VX5 is slightly brighter than the Nightforce but not by much. The VX5 makes the VX3s look terrible in comparison. There was 7.5 mins of light difference on the day I tested the VX3s and VX5 with the VX5 trouncing on the VX3. The VX5 and Z6 were very VERY close. Perhaps there was a very slight edge to the Z6, but the firedot reticle is so good that it makes it a much better scope in these conditions.
Again I want to stress I'm ONLY talking about low light clarity and visibility, not hardiness, tracking etc. SIMPLE low light clarity.
I bought a VX5HDi