I think that if it is necessary to setback the timeline, the best measure is a plague. Nothing screws things up like a big plague, think something at the level of the black death. If you want to give it a extra punch, make into a pre-war genetically enginered weapon that got out into the wild, and it was created for maximum efficiency regarding lethality-contamination.
I can even see it here: The factions with the capability to make cures or vacines either reserve them for themselves, keep the good stuff to their elites and necessary underlings, or sell to other rich and powerful people in exchange for money and influence. Super Mutants smugly laugh and pass by the piles of moribund people, muttering about their innate superiority to the weak little ones. Ghouls get attacked because some think they're plaguebearers or typhoid maries. Many people scam other dudes with crackpot snake oil cures. Psychos and thugs infected go on all kinds of rampages, take all the drugs they can and rape everybody they get their hands on in a attempt to get the biggest high before death. Scavengers swarm any unexplored facility with possibilities of containing any cure or vaccine, with predictable results. Refugees flee and cause chaos and upheaval, spreading plague and trouble where they go. People even shun public gatherings due to fear of getting contaminated, and personnal contact is a rare thing reserved for intimate friends and family. People become less intereted in casual sex, and even prostitutes experience a fall on profits. Many communities isolate themselves. Some take to the custom of using body-covering clothing and gas-masks.
Damn, I think I liked my premise. A lot.