Sinica had a historically dense population which the Miasmata failed to competently reduce, so there always ended up being plenty of people to (shakily) rebuild, but not always enough preferred land for them. The high population demanded the reduction of the individual in the society in order for stability to remain. Rome's interactions provided a catalyst for new ideas, including the ideology most similar to OTL communism. The densities of people, domesticated animals, and rodents contributed to plagues and sickness, but just as many radicals as drones were killed off. For centuries the Tang and Nanchao had each managed to control or eliminate the various radical elements much like Rome did, but by luck and trend those radicals eventually managed to topple systems that remained too rigid.
The Nanchao population was largely made up of ethnic Bazu(Bai) and Ni(Yi) along with many others which migrated to Rome's Magada vassal(~Bihar). By 1900(1147) Sinicans outnumbered Indians in Magada, and Magada was released as a vassal to become the seat of the Nanchao re-conquest of Sinica. Magada's numerous copper and iron mines were a great asset to the Nanchao.
The Nanchao adopted Rome's practice of basing the year on the foundation of a city, they based it on the foundation of Xin Darlit(~Dali), founded in 1694(941), so the date 2193AUC would have been the year 499.
After the Nanchao asserted control over the Indo-Sinican nations of Pagan and Assam in 1913(1160) confidence in the Nanchao dynasty lead to greater investment by Rome. By 1928(1175) there were three times the number of merchant ships (than before the collapse) carrying six times the amount of cargo (larger ships), and the first vaposcurr lines were built.
It was during the Nanchao era that Roman cultural influence was greatest, however the Nanchao did react against Rome's social strata breakdown, if not as strongly as the Tang had. The laxing of the caste system wasn't enough for the rebels of the twenty-first century.
Romans were called Tats'in by Sinicans and much of the Orient.