I really think that Armour is an equipment option that could realistically be included in both a single-player campaign and multiplayer game based in The Last of Us universe.
This would mostly be things you obtained from the black market in QZs or from trading posts and markets in secure settlements. For example:
- Slum-Flak - Basically two-layer canvas clothing stuffed with things like cork panels and multiple layers of rag in between the layers. This has the effect of reducing the damage you receive from body hits from weapons by about a quarter and melee hits by about a half until it degrades after about eight or so hits;
- Slum-Plate - Made up of bits of metal (like corrugated iron fencing), crudely shaped to fit around the body; it's heavy (and, in multiplayer would reduce your carry capacity a bit). It also slows sprinting speed and increases the likelihood a Clicker would hear you. However, it provides 50% protection from all attacks; having just Slum-Plate arm braces would make parrying melee attacks a lot easier. Like Slum-Flak, it degrades with time, being good for ten hits.
Given its nature, this crafted armour would be upgradable and either craftable or repairable through the crafting window and with the right training manuals (adding extra trauma plates to vulnerable areas and shaping to buy off the agility penalties).
If you have the guts to field-strip dead Hunters, infiltrate their camps or go into the Infected-controlled hells of old National Guard and Police buildings, you can obtain Police Body Armour, which give 75% protection against melee and 50% protection from ranged attacks from pistols and shotguns. This would have a durability of 25.
The top form of armour would be FEDRA-standard Military Body Armour, giving 75% protection against all attacks in all covered areas. The big disadvantage is that Fireflies and just about everyone would mistake you for military and respond accordingly. This advanced kit would be good for 30 hits.
Armour protection would be shown on the HUD as a second arc outside of the player character's health statistic, showing the remaining durability.
Additionally, the game would modify durability by the effect of the attack - a shotgun blast at close range could take off ten durability points whereas a single assault rifle shot at extreme range would take off only one. A good 'one point' baseline would be a single strike from a Shiv or a 2x4.