What book and page can I find the Vulnerability critter weakness in? I looked in SR4, Runner's Companion, Augmentation, and Street Magic and couldn't find it.

It isn't anywhere. They forgot to put it in.

The recommened handwave is to treat it as a severe allergy for the time being.

Although that isn't exactly useful as some creatures with a Vulnerability have a Severe Allergy already.

What it is supposed to do (as said by Ancient History, I think, somewhere on these boards); damage caused by something a creatures Vulnerability cannot be healed by Regeneration. To clarify, damage caused by an allergen can be Regenerated, once the allergens presence is removed.