So here's my question.
Let's assume for the moment that hong and Andur are correct - the [W] dice maximised by the critical are considered to have been rolled, and thus trigger the Property line in Vorpal.
We 'roll' our initial 2d8, and due to the magic of criticals, they both come up 8. Property kicks in, and we get to roll another 2d8.
The results of these dice are added to the damage total, and they are not "damage as the result of a critical hit" - I'd have gotten to roll these two dice again whether or not I'd rolled a critical, as long as they came up 8s. Since they are not "damage as the result of a critical", they are subject to maximising effect of the critical, and they both come up 8s. Which means Vorpal kicks in again, and I get to roll 2d8 again, and I get two more 8s.
Result? Just like in 3E, a critical with a Vorpal weapon kills, since the critical deals the maximum possible result - infinite damage - plus 6d12.
-Hyp.