There are several basic considerations - burst damage vs. sustained damage for the weapons, tanking or regen for the shields. I never go for a pure armour tank because your modules, even with MRPs, are very exposed, particularly your engines and hardpoints. Hybrid builds can work, provided you keep at least some charge going to your shields when they go down. If you have enough hardpoints, it's quite possible to do a mixed build with a respectable trade-off between burst damage and sustained damage.
The key considerations for the Viper Mk. III are weight and power. As for weight, it's a bit chubby (even after 10t was shaved off its hull mass) and you need to make sure you keep the weight below optimal or you lose fistfulls of engine performance. With regard to power, like the Vulture, it's initially restricted by an anaemic power plant. For more bread-and-butter builds (small beam lasers and medium multicannons, for instance), this is fine with a bit of power management. It's not for more power-hungry builds, however.
I like plasma accelerators and rails on my Vipers. Engineering is, therefore, a must.
Engineer the plasmas for efficient (as high as it goes - G5 - for power and heat reasons; the extra damage is nice too.) Target lock breaker and dispersal field are useful secondaries, as is flow control if you have power issues. It's not really viable, in my opinion, to put long-range or focussed plasmas onto the Viper: you will cook yourself. I mean, if you like your cockpit being on fire, it's fine though. Mod the rails for long range (G3 is fine - are you really going to engage a ship more than 5km away?) and take the super-penetrator and feedback cascade specials, as these reduce the heat output by about 40%, iirc. The reasoning behind this is that you'll not only feel like a badass when you kill stuff with all fixed weapons in FA-off, but they're also high burst damage weapons which are useful for FA-off combat. The Viper is small and fast in straight lines, but doesn't have fast rotation speeds. In fact, it's rather slow - quite close to Python performance in that regard. This, however, makes it easier to aim with fixed weapons as you're not twitching all over the place constantly compensating because you're turning too fast. It has excellent acceleration from the translation thrusters.
Take a G3 overcharged powerplant with thermal spread. I don't think, off the top of my head, that you'll get away with armoured G4/G5 within the power budget. You need the extra power for the weapons and the drive modifications.
Thrusters are G5 dirty drives with the drag drives special. The acceleration is only rivalled by the Orca, another speed demon, and you should be able to hit 575m/s without issues. For giggles, pretend you're a tuna spearing into a baitball.
Shield is a 3C biweave modded for thermal resistance with a fast charge special effect. The shields on a Viper are going to be weak no matter what you do, so pumping the resistances reduces damage taken while the fast charge brings your shields back faster, either from broken or charging. No SCBs. They use too much power and are pretty pointless on a biweave. The best defense is to not get hit in the first place. Shield boosters are whatever grade you can fit within the power budget engineered for thermal resistance with flow control secondaries, or if you can afford the Mw, further tweaks to resistances.
Armour is G5 heavy duty on the standard lightweight alloys with the deep plating special - military grade, reactive and reflective armours all take the ship over the optimal mass, so you lose performance. The ship should be fast enough to outrun anything except other highly engineered, high-speed vessels, like the Courier or an Orca. NPCs will pose no real threat unless you decide to stick it out with your shields down and they start spamming missiles at you.
My build is
here.