So "2d4 does less than a 1d12 weapon without vorpal. 2d4 does less than a 1d12 weapon with vorpal" is important, but "2d4 does less than a 1d10 weapon without vorpal. 2d4 does more than a 1d10 weapon with vorpal" can be glossed over?
Either the progression is meaningful and must be maintained, in which case the reversal of the 2d4 and d10 is a crime against the system that must be remedied, or else the progression is a point of trivia that has no relationship to the rules.
Since there's no interpretation which actually maintains the progression both with and without vorpal, it appears the second is true, and examination of the progression gives no insight as to how the rules apply.
-Hyp.
Dang it, I just wrote a long post that explained everything really well. And then it got messed up somehow. a;djfk;ladsjfj23rerwjfoiafj
Ok, let me try to recreate. I found an explanation that character maintains the relationship between all weapon die types as far as > and < at base, with GD, with Vorpal, and with both.
For weapon die entries like 2d4 or 2d6. You only explode on the highest possible result, max damage die, an 8 or a 12. With GD, you reroll each one rolled, but this only leads to explosion if the result is max. So for 2d4 with GD and Vorpal, you only explode if you roll (4,4), (1,4) and the 1 rerolls to a 4, or (1,1) and both reroll to 4s. If you take this interpretation, then all weapon die types are consistent as far as > or < except for two exceptions. When you use GD, the average result for 2d4 and d10 are identical, however d10 has a higher max damage.
But when you use GD and Vorpal, 2d4 and d10 are still identical, but there is no such thing as max damage. Although, it is less probable for 2d4 to result in a very high damage number than it is for a d10. With GD and vorpal, both types will explode 1/9 of the time if you only reroll on an 8 for the 2d4, but each time the d10 explodes it adds its new roll to 10 damage, while 2d4 adds to 8. Yeah, potential damage is infinite for both, but there is a greater probability for a high number of non-infinite damage for the d10. Take this for example. For 2d4, 72/81 of all rolls will result in 4-7 damage, 8/81 will result in 9-15 damage, and 1/81 will result in 16+ damage. For a d10, 72/81 of all rolls will result in 2-9 damage, 8/81 will result in 11-19 damage, and 1/81 will result in 21+. This can be extrapolated infinitely. So I don't really see a problem.
Damage probabilities for 2d4 and d10 with GD and Vorpal:
89%
2d4 4-7
d10 2-9
9.8%
2d4 9-15
d10 11-19
1.2%
2d4 16+
d10 21+
So, on average, these two types of weapon die will do the same damage, but it is more likely that the d10 will do a large amount.
FINAL TABLES:
Base average
d4 2.5
d6 3.5
d8 4.5
2d4 5
d10 5.5
d12 6.5
2d6 7
GD average
d4 3
d6 4
d8 5
2d4 6 rerolling all 1s
d10 6
d12 7
2d6 8 rerolling all 1s
Vorpal average
d4 3.33
d6 4.2
d8 5.14
2d4 5.33 rerolling only an 8
d10 6.11
d12 7.09
2d6 7.2 rerolling only a 12
Average with Vorpal and GD
d4 4.5
d6 5
d8 5.83
2d4 6.75 exploding only when a 8 is rolled on 2dice but rerolling all 1s
d10 6.75
d12 7.7
2d6 8.33 exploding only when 12 is rolled on 2dice but rerolling all 1s
So, there is an interpretation which actually maintains the progression both with and without vorpal. The progression is meaningful and can be maintained as I said above.
Regardless of whether or not you like this interpretation or think it is correct, it is the only one that maintains the continuum of average damages for each case: d4<d6<d8<2d4<d10<d12<2d6. It is important to note that because 2d4 and 2d6 are considered a weapon die each set must be kept together when rolling multiple [W]s and they can only explode when an 8 or a 12 is rolled.
In summary, any and all 1s can be rerolled when using the GD, this does not break the weapon damage die progression. GD are not the problem. Rerolling every 4 or 6 when rolling 2d4 or 2d6 damage does break the progression both with Vorpal alone, and with Vorpal and GD. Rerolling 2d4 and 2d6 only when an 8 or a 12 results does not break the progression. Therefore, if you feel that this progression is an important system structure that needs to be maintained, the previous is the only rules interpretation that is valid.