Step 1, find a smersh vest/backpack in any condition.
Step 2, find the counterpart to the first item, combine them.
Step 3, detach the backpack from the vest when taken damage and attempt to repair.
Step 4, find pristine or worn vest/backpack and attach it to the counterpart until you are able to repair the backpack or replace the backpack/vest.
Step 5, detach backpack from vest and notice your previous worn/pristine item is now damaged as well.