If Vileplume's "Allergy Panic" Ability is in effect, does Rayquaza's "Shred" attack consider a Defending Pokemon's weakness as x2 or x4?
Shred isn't affected by "any effects on the Defending Pokemon", but the term "any effects" does not include effects that alter Weakness or Resistance. So in this case the Defending Pokemon's Weakness would be x4 due to Allergy Panic.
Source: TPCi Rules Team (2013-02-28), TPCi Rules Changes Announcement (2017-05-03), TPCi Rules Changes Announcement (2017-05-18)

Relevant cards

Note: All applicable cards may not be listed below.
BW6_128
BW7_3
DV1_11