No you will not need the 20 moa rail to get to 1200 yards with that set up. But later if you want to go farther you probably will wish you had it. A 20 moa rail will allow a 100 yard zero and add that 20 moa back to your elevation above the zero. The Viper is not long on internal elevation so I feel the choice of a canted rail would be good.
So, here is my advice. Rails and rings are definatly a "get what you pay for" item. A good rifle and scope can be degraded fast with poor quality mounts. I would spend a little more for a solid platform that will insure no movement even after 100's of rounds are fired. I will suggest you go for the Nightforce 20 moa rail and a set of the low (.885 or 1.0) ultralite Nightforce rings. These are quality items at affordable prices compared to the cost of the rifle and scope. You have a good start to a long range rig, so keep it going with some good mounts.
The NF rail and rings for a long action REM 700 is something you will be able to use for many applications for years to come. So these mounts could be with you for a very long time even if rifles or scopes are changed. Also if you decide you no longer need them they are very sellable as used mounts and sought after. So resale prices are good and your investment is protected.
Jeff