I've been swamped with things and very lax on getting my reviews out. My apologies.
I played with a Vortex Viper 3-9x40 with a regular plex reticle. I was able to compare it to the bunch of scopes that I have and shot it on my 308 (a reasonably heavy rifle with mild kick). In this picture, it is in the fron row second from the right (not a very good pick, I know). Right next to it is a Sightron S2 3-9x42.
I also spent some time looking at the optics at home in different lighting conditions.
To me it looks like a previous generation Nikon Monarch 3-9x40 with some cosmetic changes (mostly a different magnification ring).
It is about on par with Sightron S2 3-9x42 that I had on hand. Both scopes resolved about the same level of detail in good light and in bad light. Viper was a touch more forgiving in term of of axis light coming into the scope, but that may be because its front lens is set slightly deeper into the objective bell than that on the Sightron.
The particular Viper I had exhibited a small change in POA through magnification changes, but nothing particularly out of the ordinary (<1" at 100 yards). For a hunting scope that would not make any difference whatsoever. Adjustments were pretty good and quite repeatable. I did a box test without any problems.
Viper has a fast focus eyepiece which is an advantage for some people and easily resettable pop-up knobs.
All in all, it is a solid hunting scope. The problem lies in the value proposition. If memory serves me right, Viper 3-9x40 runs around $300. Sightron S2 3-9x42 runs around $200. Bushnell Elite 4200 3-9x40 runs around $260.
Elite 4200 has slightly better glass. Would you pay more money for a Viper with pop-up dials but slightly worse glass?
Sightron S2 is substantially cheaper. Would you pay an extra hundred bucks for fast focus eyepiece and pop-up dials?
I would not, personally, but it really comes down to what is important for you.
Another note: Sightron S2 Big Sky 3-9x42 is in the same price range as the Viper, has similar pop-up knobs, better glass and more eye relief.
Vortex Viper 3-9x40 is a nice scope, but I am not sure why I would buy it over the competition.