I have to say I disagree with Hong's ruling on a 'damage roll' and 'rolling damage' being one and the same.
To me:
Damage Roll = Total damage done, despite any actual rolling
Rolling Damage = Physically rolling for damage.
A minion's damage could be considered a 'damage roll', for example, but no one actually 'rolls damage'. Similarly, a critical hit damage can be considered a 'damage roll', but no one 'rolls damage'.
I admit it's somewhat bad English, and we can debate the meaning of 'damage roll' to the end of time and back. But when someone says 'X happens when you roll damage' and a critical hit says 'instead of rolling damage', it seems pretty obvious that this does not happen on a crit.
This is just RAW, as I see it. Not RAI or RAU or whatever. With Wizards and DnD, especially with their new 'keyword' system, it seems a lot of things need to be taken literally, unless they make little sense. As others have shown statistically in this thread, a vorpal weapon still does plenty of damage even if you don't reroll [W]s on a crit, so it seems like this is RAI as well.