I have almost the same thing happening with my Viper pst also. I also have a 20MOA weaver tactical base on my winchester action with a kreiger barrel on it. I have a grand total of 24 moa of upward travel. I am thinking it is the base or the rings that I have on it, they too are weaver rings as I didn't have the money at the time to get good stuff but now I wish I wouldve. The reason I say this is I did have a Leupold LR one piece base with the standard rings before I put on the weaver stuff and had 36 moa of up travel. So long story short, I am going to ditch the cheap stuff and buy NF, you do get what you pay for.
On another note, not too impressed with the vortex pst