I'm just opening this discussion thread to repost the following:

MrViciao Oct 6, 2018 @ 8:49am
For this mod to work with the vest slot mod, set 'ITEM_CATEGORY=defense' in the ini file

1) For those who aren't familiar with making their own ini edits, the mod folder is found here:
D:\Games\SteamLibrary\steamapps\workshop\content\268500\
(Your drive and starting folder may vary depending on what you chose on install)

2) This mod's folder is 1318532978 (when browsing Steam, as you are right now, you can see the mod number up at the top of the browser right below your username).

3) Open the Config folder, then open XComWOTC_LW2_Plating.ini

4) Make the change described by MrViciao above. The note in the file incorrectly states that changing the setting to "vest" will work, but it doesn't. Changing it to "defense" most definitely does though.

[WOTC_LW2_Plating.X2Item_UtilityPlating]
ITEM_CATEGORY = defense

Works like a charm. Thanks MrViciao.