If you fill up your inventory so you dont have any space left, put extra assault vests or high caps in your backpack/jacket/pants, you can double click on items on the ground and the items will automaticly go inside the vests. That way you can carry more items on your character.

If the vest is on the ground, you can not put stuff inside it and then put the whole vest in your inventory. You have to put the vest in your inventory and then double click on items on the ground for it to be stored inside the spare vest in your backpack.