Realized with the gamified PvP conversation idea I was dimly thinking of a system like the one Klei is working on for Griftlands -
https://www.klei.com/games/griftlands - cards, or some equivalent, could work well.. maybe you find etiquette books scattered in the world that give you "lines" and ways to relate/emote
You could also do a kind of grid-based tug of war or "Battleship" style mechanic where different conversational gambits work or fail, as you're trying to discover information about each other (Stats, inventory, etc.).. and/or maybe each player has meters based on real-time stats (fatigue, radiation) and their reputation stats, like, "relaxation/annoyance/irritation/anger", "interest/curiosity/boredom/indifference", "stinginess/skepticism/openness/desire/greed", that you try to shift towards one end while keeping your own away from the other..
Just... something.. other than completely unstructured chat or pow pow pwned as the only programmed gameplay options..
Like, multiplayer interaction is cool in theory, because one of the most "Fallout" aspects of the brand is the sense of moving through a dangerous world and interacting with characters who are more or less trustworthy, with their own motives.. if you structured a way to enact that dynamic with real people, you could make the game exponentially more interesting..
Like, maybe encountering someone outside a hub, the options are "Hail/Wave/Ignore", you can also ignore, or "insist" to someone who ignores you, wave back or wave away (politely acknowledge/decline), or hail back/respond, or duel, all with impact on reputation stats - if you both "hail", you can like, warn/threaten or challenge, welcome or gift, propose trades, item requests, contracts, and opt in to fight or to "parlay" - and "parlay" introduces the conversation/negotiation game/'combat' mechanics, where you can put any of your items, item requests, caps, cards, weapons, stat points, contracts ("help defend x", "fetch fifteen y"), quest requests ("go clear out place x") in play (maybe you can specify some of your inventory as "off the table", some on the table, and some unknown - available but not visible) and then try to entice/bait/outnegotiate each other into getting none, some, more or all of what you want - guessing about the unknowns, playing emote/talk "cards" or moves, revealing information about your unknowns, and giving away some in order to move the other person's meter.. and maybe there are nuclear options (rob, cheat, try to change deal, surrender/fold/exit early) that have moderate to big reputation hits (and impact your stats negatively the next time you parlay with someone)
Well-balanced and meshed with the gameplay, not its own huge heavy thing, it could add another layer to peer interactions, another opportunity to "create stories"