If you crit with a spell/attack you roll double the dice you normally roll. What happens with this example?
Vicious Weapon
When you roll a 20 with this magic weapon, the target
takes an extra 7 damage of the weapon's type. [Note: According to the
SRD, it is an extra 2d6 damage.]
(SRD 2d6) are these always 2d6 or you have to double them to 4d6 'because it is a critical'?
Best Answer
You roll any dice twice, but add flat modifiers only once
As the basic rules describe:
Any damage expressed as dice gets to be rolled twice, but other modifiers are added only once. In the case of a vicious weapon, since the damage bonus it grants is a flat number (7) rather than bonus dice, it doesn't deal any extra damage on a critical hit.
If you're using the version of the vicious weapon found in your SRD document - which adds 2d6 damage, and seem roughly congruent with the version in the published books, as 7 is the average of 2d6 - you would indeed double those 2d6 bonus damage dice to 4d6 if you critically hit, just like you do with sneak attacks or smites.
Which version of vicious weapon is correct has been discussed before - according to the developers, what's actually printed in the books/core rules takes precedence over any content in the SRD:
My guess would be in this case that the content of the SRD was an earlier version of the effect, where the intention was a for a weapon that got to do +2d6 bonus damage on a critical hit, before it was realised that a critical hit would also double these bonus damage dice and make the ability more powerful than intended (as well as being confusingly worded). Rather than write an exception for this edge case (by clarifying "This bonus damage is not rolled twice because of a critical hit"), I suspect they chose to simply change it to an equivalent flat modifier instead, so as to remain consistent with the general rules about critical hits.