I always preferred the "explosive critical" version of Vorpal. it goes well with my mental vision of "Vorpal" being some sort of vibroblade feature or something.
first, all critical multipliers are increased by 1; X2 to X3, X3 to X4, and X4 to X5.
second, when you roll a natural 20 (rather than just a hit in the crit range) you roll twice to confirm. if either confirms, you roll damage. if BOTH confirm, you roll damage as if for two critical hits. if you roll a 20 on either confirmation hit, you roll two more confirmation hits.
so, assume a +1 KEEN IMPROVED CRIT FAVORED CRIT OMGWTFBBQCRIT rapier which threatens on anything from 5-20. the weilder is fighting a Doorstop of Sucking -5, with an AC of too-low-to-matter. he rolls a 7, hits, and confirms the crit with a 5. roll 3d6+3 for damage. now consider if he rolls a 20 on his threat; he rolls two confirmation rolls, and gets a 10 and a 17. he then rolls 6d6+6 for damage.
now he threats on a seven, confirms with a 20, and then rolls a 1and a 20. he continues spreading upwards, branching his confirms until he stops gtting 20s. on average with this weapon he'll average about 4d6 + change on a crit threat, rather than consistent instant death. it still allows the possibility of a 10,000 damage crit, but it makes it vanishingly rare.
I saw it in the House Rules forum here, and i really do love the idea a hell of a lot more than "automatic head-lopping".