A vorpal confirmed with another 20 works like it used to against non-purple/reds.
If less then 1000HP, a confirmed vorpal kills.
If more then 1000HP, a confirmed vorpal does 10% of HP damage. Against an enemy with 1001HP it would do 100 damage, 2000HP/200dmg, 5000HP/500dmg, etc, etc...
As the fight goes on each vorpal will do less damage then the previous, until the enemy is under 1000HP. Even if you are on a crit-streak, chances are that some people will never see a double 20 insta-kill. But if it happens at the right moment, it'll be a high5 occasion.
That's my 2cp