The way it should be handled ( sorry if I get my terms wrong ).
Is that the leveled item lists require you set the equipment to randomly generate from those items held within it.
They are set to appear at a specific level from one onward.
The basic items in the vipers should spawn at level 1+.
Those extra, the ripper the 10mm smg, and lever weapons, randomly at PC level 8+.

So you get one piece of armour ( merc or metal in vanilla ).
One weapon a 10mm or melee normally at under 8, or above 8 the better options as well.
Then all the misc, chems and caps.

Edit: an uneducated gusss is that the base vipers themselves are set at level 7...
So unless you make new versions to spawn at higher levels or set them to level up with you, their weapons won't upgrade.

Well that was easy..

The answer is that vipers are just set below the level for better equipment to spawn.
So for them to equip better guns you need to increase their level in the GECK.

NV seems to have almost removed leveling completely by just setting every npc at one level, in the vipers case that was 7.
Which makes the game progress in difficulty as you follow the main quest, just limits the groups you encounter to a limited weapon set.

This may be why many find it easy to play.
Later on you can face tougher foes, but those you face early on never change even a little no matter how many times you face them.
Making it less of a challange going back over trodden ground.

I think many hated Oblivions style.
That made it seem a better choice to not level at all the game world, outside of the MQ.