I called Vortex today, but I posted here first because I wanted opinions from this site.
I asked if the glass is the same, and the response was "well, it depends". Then I specified exactly which two scopes (Diamondback HP 3-12 vs. Viper HS 2.5-10). I never got an answer as to whether or not the glass is the same, but I suspect the answer is "yes". Their representative said they would be very similar, and they "are both great performers". I didn't push him to give an absolute answer, as he clearly didn't want to. I think I got my answer indirectly.
I then called Steiner, and asked the same question about their GS3 2-10 vs. Burris Fullfield E1. This time, it was confirmed that the glass is different. He claimed the Steiner has German glass, while the Burris is of either Philippines or Japanese origin. He was, however, reluctant to say that the Steiner would perform better in low light conditions. I was told the coatings on the Steiner may be slightly better, but that most people would probably not be able to notice any difference.
I'm not sure if his reluctance was because he didn't want to sound as if the Burris was sub-par, or if the two scopes really are as close as he says they are.
Maybe I just need to buy a Conquest HD. I looked at the Sightron Big Sky, but I really don't like the reticles.