I have only about 2000 points to spend on weapons at the moment. I hear the Vipera is pretty good but I'm wondering if I should buy it or save for something better. I am speced to use Assault rifles at the moment but am willing to respec if the weapon is worth it.
Ignore the vipera; get the shotgun. Put points in as a medic and get in close to the action where the shotgun reigns supreme. XP up until you can afford the AK or AG. As Bjorn stated, the vipera is a good stealth weapon but you don't have the money for the sensors and suppressors at low level so direct action is a better tactic for the early game.
I bought the AK at low level and soon after switched to the Vipera and still use it at 70.
Vipera + Suppressor + Sensor Jammer = Ninja mode.
I notice that many people don't regard me as an enemy until I pull out the knife, at which point it's too late. Fun play style and easy to get behind enemy lines to destroy assets.
Make sure to place your points properly into the personal defense tree for improved stealth and weapon use. Vipera is classified as sub-machine gun and thus falls under Personal Defense.
The bonus of the Vipera is the large clip. The downfall is the stopping power. However, learn to aim at your enemies head area and you can easily take down groups of 4-5 with one clip.
I never aim for anything but the face when using the Vipera.