I'm not quite sure what the number should be but 1000 HP feels too low. Bad-builds with junk gear will put out 1000 points of damage in less than 20 swings. Even good vorpals like the fully-upgraded Brigand's Cutlass aren't worth using as the insta-kill procs so rarely you're better off with lightnings.
I guess if you don't have GS yet vorpals are fine but if you've got lightnings/ESoS already it doesn't look like there's any point in using them from what I've observed in the past few days.
I don't think they should kill a non-deathwarded target regardless of HP, vorping that 76,000 HP giant in claw would be silly but 1000 HP is too low of a threshold. Not sure what the right number is, but that's too low.