Confirming a crit doesn't mean needing to roll a second Nat 20 (which I agree would be rubbish), but rather it means you make a second attack roll and if you were to hit again then the crit is confirmed.A 1 in 400 chance of decapitation is a bit rubbish. Maybe make the confirmation roll 'your level or less on a d20'.
Also base vorpal swords (even without the confirmation roll) are relatively weak - I suggest having swords of sharpness and vorpal swords always deal maximum damage on each hit (ie. 8 on a d8).