I remember a knife in fallout 1,2 - Shiv, it's called- with a description "operated like a switchblade". So I think, What's about Shiv with two option. equipped on hand, if left alone the stats of that hand look like bare fist (or foot), attacking by that hand will using Unarmed skill; shiv Switch On will act like a knife, a small percentage to cause sudden death, later on a chance to coat poison on its edge. If we can kill the victim in one turn and switch off Shiv, the guards and everyone else will not see anything, but run around a bit.
And may be a another weapon described as equiped on foot for silent kill.
And may be a another weapon described as equiped on foot for silent kill.