You do realize the historical implausibility of this? Southeast Asia rarely had fully functioning empires, and when it did, they were normally just stretched to capacity keeping control over their large, multi-ethnic, and religiously diverse territory. The odds of a Kingdom uniting all of India or Southeast Asia to take on China is... preposterously unlikely. It would be akin to saying: "what-if... the Armenians after world war one combined in Syria to take over the Middle East or the Maghrebs in Algeria combined and takes all of Africa, every last territory, from the European powers."
The only real plausibility of this occurring, at all, would be with Tibet, during the time of the Tang, when the two nations were rival empires in terms of size. Population and resource wise it would be nearly impossible for them to take on a functioning empire. Which would lead to a powerful Tibet taking advantage of a fractured China (assuming it avoided it's own fracturing problem and it's crushing defeat by the Tang) and later moving itself to Chang'an or Xi'an or perhaps even Luoyang to manage the empire. Even this, would be nearly impossible. The Qing and Yuan Dynasty only were able to complete their conquests of China by employing the Chinese themselves. The Mongols were having trouble with the southern Chinese and required many northern Chinese defector generals to lead the advance south under Mongol leadership. The Qing were similarly helped along. Also here's a kicker, the Chinese troops that conquered themselves were kicked out of China by Kublai Khan and used in Persian and Arabian campaigns, to great success.
So: You can't conquer China without the Chinese.
And despite saying all of that against this. You can count on my submission.