The Vampiric Fury Shortswords do reduce CON saves, but I can generate a good Freezing without them much of the time with my higher WIS.
The Envenomed Blades are better for bosses since they also have armor piercing. I don't rely on its paralysis effect but they are excellent to accelerate poison damage since their untyped poison damage is magnified by the Ninja Poison debuff.
My
last post compared Assassins to Ninja Spies. The "poison" attacks called Poison Strikes for Assassins aren't true poison damage, as found in my tests, but a debuffing effect. The class still gets Venomed Blades for true poison damage, and have some attack versatility advantages over ninjas with the right training.
What? You didn't like the pun on the Method? Okay, okay, I'm still working on it....