"the yarlung dynasty, invades persia, defeats the arabs and places yazdegirid back on his throne as a vassal. his son peroz converts to buddism and it takes off from there."
The problem is, Yarlung is in no condition to expand, at the time it was still a immature empire whose influence is well out of the political scene of central Asia. And Buddhism is not the state religion at the time so there isno reason to suppose that Yazgerd will convert to it. Even when Yarlung is at its height, it did not have the potential to hold on to its conquest as history demonstrated its insecure possession of southern Tarim as well as almost no control over the Turgis.