Because Vorpal Blade delivering random, unpredictable instadeath is not fun to be on the receiving end of, replace it (or just replace its instadeath effect) with a “chaos blade” that has a bunch of random unpredictable effects, including some that may backfire on the user.
Such a new artifact would almost certainly be chaotic rather than neutral.
Wielding Vorpal Blade in your offhand will never decapitate monsters; instead hits that would have decapitated will instead sever a limb, which can make the monster lose its weapon or permanently lose a claw attack.
Invoking Vorpal Blade gives some sort of message about its keen edge gleaming, indicating that its next hit will have a high critical ratio (perhaps 10% instead of 5%) for increased chance of decapitation.
Raise the instakill ratio of Vorpal Blade and the Tsurugi of Muramasa, but only make it work if the target has under 50% of its hit points. Thus, the player can’t be instakilled by it if they stay above that HP threshold.
Vorpal Blade (and other vorpal things in variants with them) can only instakill a monster with less than 400 HP. Otherwise, they merely deal double damage.
Decapitation is not protected by an amulet of life saving, since the amulet just falls off your neck. Alternatively, keep the life saving, but give a special message about the amulet crumbling to dust as it falls off your neck.
New object, a metal collar you wear on your neck. Makes you immune to being decapitated by Vorpal Blade. It would probably be worn in the amulet slot.
In cases where Vorpal Blade won’t decapitate because the monster is headless, it does double damage instead with the message “Vorpal Blade goes snicker-snack on the foo!”
Vorpal and bisecting weapons don’t get dull when you engrave with them.
When a monster wields Vorpal Blade out of your sight, you hear “a distant snicker-snack”.
Change Vorpal Blade to do very high damage (on the order of x3 or x4) when it critical hits, instead of decapitation, in order to remove YAAD potential in monster hands. The new critical hit message is “Your Vorpal Blade goes snicker-snack!” (Some other message might be needed to replace the existing message when you are neutral crowned but already holding Vorpal Blade).
