I've submitted this question to Customer Service. If I get an answer, I'll post it here.
The Vorpal Weapon Property(page 236) states: "Whenever you roll the maximum result on any damge die for this weapon, roll that die again and add the additional result to the damage total. If a reroll results in another maximum damage result, roll it again and keep adding."
The Critical Hits (Maximum Damage) entry (Page 278) states: "Rather than roll damage, determine the maximum damage you can roll with your attack. This is your critical damage..."
How do these two rules interact? This has been hotly debated on the ENWORLD boards (http://www.enworld.org/showthread.php?t=232887&page=1&pp=15).
Three different interpretations have come up, which one is the most correct?:
1) Infinite Damage: On a crit you determine the maximum damage you can roll with your attack, and use that damage. Since you keep rolling damage when you roll maximum damage with a vorpal weapon, the maximum damage you can roll with a vorpal weapon is infinite. So you do Infinite damage (+6d12) on a crit with a vorpal weapon.
2) If you don't roll dice, you don't Vorpal: The vorpal property only triggers when you are actually rolling damage dice, not when you are determing the maximum damage from an attack. So you do your normal maximum damage with that attack +6d12 and the vorpal property would kick in on the extra 6d12 damage dice your roll.
3) Vorpal activates for "free" on a crit: Since you use the maximum damage roll on a crit, your vorpal property automatically triggers once for each damage die you would have rolled, but you still have to roll the extra vorpal dice to see if it activates again. So an attack with a Vorpal weapon that does 3d10+15 damage would do 45(max dam)+6d12 (extra crit dice)+3d10 (extra vorpal dice) damage, and the vorpal property could trigger again on the 6d12 or 3d10 damage dice when you roll them.
Thank you for taking the time to answer this.
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Page Number: Page 236 and Page 278
Book Name: Player's Handbook (4e)