If I use Cresselia-EX's "Psychic Protection" attack, does Vileplume's "Allergy Panic" Ability override and give it a x4 Weakness instead?
No, since there is no longer a weakness there is nothing for Allergy Panic to multiply.
Source: Boundaries Crossed FAQ (2012-11-08), TPCi Rules Team (2012-11-08)

Relevant cards

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