The Silencer on the Desert Vulture is pointless, you'll still pull zombies in a room and spawn zombies in the Wasteland, unless TFP fixed it. Only found the silencer effective on the 9mm Pistol and SMG, I dunno about the sniper rifle.
I would go - Mag Extender, reflex sight, cripple them and Rad mod.
If I had to just pick 2 - Cripple them and Rad mod ... on allll guns.