There are several tutorials on modding almost everything else in game EXCEPT carry items like loot and vests. I've tried many times to figure it out myself and use examples from mods, but haven't been able to make anything appear in the game's armory.

jI can't speak for loot but this is for vests. Also note that I am just an amateur modder and I've figured out what most of these do through trial and error.

First of all, make sure that the vest you have modded is labled in the [all_carry_items.xml]. You may also want to add it into the faction [.resources] files if you only want certain faction(s) to have access to a vest. Or just add it into the [common.resources] file and make sure its enabled and put in correctly.

More advanced vest functionality:
[transform_on_consume="vest2_2"] means that on initial equip, the vest will transform your character to whatever it is labeled under in the [faction.xml] file.

Example:
<model filename="soldier_a4.xml"><requirement class="carry_item" slot="1" key="titan.carry_item"/></model>
<model filename="soldier_a4.xml"><requirement class="carry_item" slot="1" key="vest4_2"/></model>
<model filename="soldier_a4.xml"><requirement class="carry_item" slot="1" key="vest4_3"/></model>
<model filename="soldier_a4.xml"><requirement class="carry_item" slot="1" key="vest4_4"/></model>
<model filename="soldier_a4.xml"><requirement class="carry_item" slot="1" key="vest4_5"/></model>

Here you can see that in order for any soldier under the faction tab to use the model labeled [soldier_a4.xml] the actual vest # also needs to correspond. [titan.carry_item] donates which vest will be used in the items folder. However in order for the vest to actually absorb the number of hits you want it to, it requires the ["vest#_#] keys afterwards. These are for the vest to know when to stop absorbing hits or when to be used up.

Continuing with my Titan vest example:




For the most part, the lines before [<!-- death or wound -> stun only -->] or [<!-- death or wound or stun -> no effect -->] are the ones that are changed the most.
So the first sort of group of lines in vests should look similar to this:

- your [<carry_item key=] should always be the name of the file.
- name is self explanatory
- slot should be left alone
- [transform_on_consume=] should always be labeled as ["vest#_2"] From personal experience, I've never tried changing the second digit to 1 as I simply followed the way vanilla labeled its vests. I am also not 100% sure if changing the part labeled [vest] affects anything, but from my experience it resulted in the vest not working/showing up (most likely just an error of some sort on my end).
- you should also refrain from using "2" as the first digit in the [transform_on_consume=] line unless you want to replace vest2.
- [time_to_live_out_in_the_open="120.0">] is just the amount of time in seconds the item will stay in the world.

- [hud_icon filename=] self explanatory
- [<capacity value="0" source="rank" source_value="0.0" />] and [<capacity value="1" source="rank" source_value="0.1" />] should be left alone unless you are sure of what you're doing
- [<inventory encumbrance=] this is how much weight in the 10's of percent the item will take up if put in a players backpack. A value of 3 means 30% of backpack space will be taken up.
- [price=] this is the RP amount to buy and sell the item.
- [<model mesh_filename="vest.xml" />] should be left alone unless you made your own model for vests when they are dropped in the world.

- [commonness value=] never really messed with this. In old forum posts they said it didn't really affect vest spawn rates but if anyone knows otherwise I'd be glad if they corrected this.
- [in_stock=] MAKE SURE ITS SET TO 1 UNLESS YOU DONT WANT IT IN YOUR GAME
- [can_respawn_with=] A value of 0 will give the item a blue font and background and will result in the item having to be bought after each death/expenditure. Value of 1 means you will always respawn with it, provided you have enough RP.

- All these lines will result in the same effect as the first bullet hole in vest2. In other words, it equals to 1 lethal hit absorbed with no knockdown to the wearer.
- [</carry_item>] this signifies the end of the previous lines for the purposes of bullet absorbtion.

The second grouping of lines is similar to the first except it is dependent on what you put in the [transform_on_consume=] section.


- Here the [<carry_item key=] should be the same as what was in the previous [transform_on_consume=]. If in the last grouping line, I put [transform_on_consume="vest6_9"] then the next grouping should start with [<carry_item key="vest6_9" name="Whatever" slot="1" transform_on_consume="vest6_10" time_to_live_out_in_the_open="120.0">] This applies to all lines afterwards.
- The rest of the lines can be left alone except for the one starting wtih [commonness value=]. Unless you want AI spawning in with vests that have less lethal hit absorbtion than maximum, [commonness value=] should be set to "0" for every line grouping after the first. The same goes for the rest of the line, they should all be set to "0" unless you have something specific in mind.

Below is the rest of the line groupings (sans the one for wounded), and the values for a soldier getting knocked down on the ground, but not wounded.


This last one is the final line grouping for the Titan vest in the XCOM mod and has the values for a soldier getting wounded and their vest breaking, thus requiring a medic.


I may have missed some parts while writing this out but I hope it helped you with your vest problems and if theres anything you need clarified just let me know.

Thanks for the help! I'll see sometime if I'll be able to get it right this time.

Is there a way to add a weapon stat modifier(changes retriggering time, sustained_fire_grow_step) to an item so if the item equips it would modify the users weapon's stats? I know you can add speed and and crosshair size modifiers to items.