Odd as it might sound to you, I disagree here. RAW nearly always contains typos or examples based on the way rules worked several revisions ago or sloppy language or things that are just plain poorly explained. We can't always know RAI, but... well, by definition, the game is intended to be played using the RAI and not the RAW.
As a quick example, on page 276 the damage type example has someone using a thundering longsword's encounter power to do 10 thunder damage and push 1. There is no encounter power in the thundering weapon description on page 236. In this case, the RAW is wrong.
I suspect that "damage die" and "damage dice" (since both the singular and plural are used, apparently regardless of how many dice you actually roll) are examples of sloppy language used in an attempt to explain what [W] means. And since rolling all your d4s in pairs in order to track which set of 2d4 rolled an 8 is too much of a pain in the butt to be believed, I'm finding it hard to accept that the vorpal power would be intended to work like that.
Plus, I've noticed that official game terms are often capitalized. Thus, halflings are Small and the items that prestidigitation can turn invisible are merely small. I don't see damage die/dice being capitalized. Although certainly not every game term is capitalized, so I'm basically guessing here, and my guess is that you are reading too much into it and treating relatively loose game terms as if they were strictly defined.
(That said, your reading of the rules isn't inaccurate. All the definitions of 'damage die' and 'damage dice' and even the weapon scaling rules on page 220 back you up on that. I'm just doubting that the vorpal rules are an intentional reference to that definition.)
One more thought:
If the vorpal weapon only lets you reroll maxed damage dice, and if damage dice are, quite specifically, [W]s... Does that mean that only the [W] part of an attack can be rerolled? No rerolling crit bonus damage because those are "dice of damage" (page 225) and not "damage dice"?