It's pretty much accepted that "looking straight up" does somewhat reduce 2nd stage airflow......the tricky bit is trying to distinguish between the 'normal' restriction versus excessive restriction caused by either poor initial factory set-up or design flaws in technology.
I've never used the Viper, but do own a number of regulators, including a servo-valve/side exhaust Poseidon Odin, which is pretty similiar in concept to the Viper.
I LOVE my Odin! (so much so that 2 days ago I bought a 2nd Odin + Odin octo 'new-in-box' on E-bay for $356.00.....which is $50.00 less than the $410.00 such a combo goes for @ LeisurePro......and it's out-of-stock there now!)
Mainly I dive either my Odin.......or my Mares Ruby or Mares MR-16 Voltrex regs.......and love them all!
Earlier this year, after I bought my almost new Ruby on E-bay I took it to my local shop for an 'annual' ...just to be on the safe side.......but when I loaned it to my brother during his Discover Scuba class in the pool, he said his air supply shut-off completely when looking up. I took the Ruby to another local shop to confirm it was adjusted properly, they adjusted something on the 2nd stage (it was a while ago so I don't remember the details....but they made an adjustment at the point where the 2nd stage joins the hose) .....and now the Ruby breathes GREAT even looking straight up.
(The Ruby is one sweet breathe.......smooth as silk, you forget you even have a reg in your mouth!)
My Odin and MR-16 Voltrex breathe great looking straight up.......there is a slight increase in breathing resistance......but both regs still provide plenty of air in that position.
So, my experience is that regs, even with rather different design technologies (the Odin and the Ruby/Voltrex differ in major ways) all experience a reduction in performance 'looking-straight-up'......but they should continue breathing reasonably well even in that position.
My 'Ruby experience' indicates the possibility your Viper is simply mis-adjusted and can be corrected with the help of a competent dive shop.
While I've never had the misfortune of buying a 'flawed-design' reg, it's possible the Viper cannot be fixed and you will be forced to suffer with the problem....I've read many reg reviews @ Divernet.com and they've had a number of regs over the years that did as your Viper does and completely cut-off the air in certain positions........they don't address whether the regs were defective designs or just poorly adjusted.......so I stayed away from those brands/models....just to be safe.
Assuming you're under warranty, I'd take your Viper back to the shop for a proper checkout......and if you have to, arrange to 'tag-along' with your local shop during it's next OW class outing......and insist that the dive shop owner/instructor dive your reg with you alongside.......to verify it's really 'fixed'.
If they refuse to cooperate, they'll be admitting their incompetence at repairing regs.....or that they sold you a defective piece of junk.
The worst-case-scenario is that the Viper design you bought is inherently flawed and cannot be fixed but must be redesigned......but I can't address that as I've never tried/owned a Viper.
If it come to the point where you want your money back, don't want to take a chance on another Viper, I'd encourage you to look at the Poseidon Odin....I can't say enough good things about it......the bubbles shoot well off to the side you so get clear vision even while stationary........and it provides excellent airflow.....you'll never overbreathe an Odin!
Leisure Pro recently had them for only $250 -new-so it's not necessarily any more expensive than the Viper and even at full retail price my local dive shop lists the Odin @ $440.00 while the 'Viper-Metal' lists for $430.00......so if you can afford a Viper you can afford an Odin.
I'd encourage you to at least test-dive an Odin.....so
you at least know what you're missing!
Karl