How's this map for a representation of what things are like after successful Chinese campaigns in Southeast Asia for the 25 years from 1765 to 1790?
The color scheme shows China expanding at the expense of states on its southern border, Vietnam, Burma and Lao states, while allying with Siam and Cambodia to help crush the states between them and China, thus in this phase at least Siam and Cambodia are not being subjugated but instead expanding.
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Southern Burma isn't really on the table, I should think. Even if some Chinese general manages to reach the delta with local allies (and given logistics, the allies would be doing the real work a la the conquistadors), the result would be a new Burmese state that wrote nice letters about being a vassal until the current emperor died. You've affected Burmese history a great deal, but Chinese borders much less. Meanwhile, the costs of extending that far through terrain that was almost impassible
during World War II creates a new Treasure Fleet situation - makes it much harder for the policy to last more than a single emperor.
The same goes for Vientiane, only more so. That's one of the most impassible routes of travel the Chinese even have access to, and these are the people who manage Tibet.
If you want to go overland for that version of the scenario, a decent rule of thumb would be the various borders of the Nanzhao and Dali states that were centered in Yunnan before the Ming. If you have a strong military based in Yunnan, history demonstrates that you can successfully contest perhaps half of the Burma on that map, a portion of that map's Luang Prabang, and probably a corner of Chiang Mai. OTL this was only ever done for more than a brief period by a couple of Yunnanese ethnic groups. But China is China, and could afford it.
If we want to explore beyond "how the Chinese could be more stubborn making the same mistakes they mostly know are mistakes and suffer for it" though, we can. That would entail going by sea. Perhaps it could be combined with more limited efforts with more limited goals in Burma - a split focus might actually make the Burmese adventure
cheaper, by keeping objectives more realistic.
The thing is - China only really firmly incorporated Yunnan on a basis capable of being permanent in the Ming dynasty. With pre-modern logistics, it was at the edge of their reach, not much better than eastern or northern Tibet (modern west Sichuan and Qinghai, respectively). Vietnam, though.... The Han dynasty took Tonkin, Annam, and part of Cochin China back in the early 2nd century BC. And China found it practical to maintain the place as a province for a thousand years.
Southeast Asian conquest is much, much easier by sea. In fact, the problem the Chinese had with Tonkin itself was not the distance, but the terrain. Distance and ethnicity wise, there's negligible difference between that part of Vietnam and places like Hainan or Guangxi. Guangdong used to extend to the Vietnamese border - why? - because it was infinitely more practical to manage the region from far-off Guangzhou than from adjacent counties in Guangxi. The trouble with Tonkin is that while most of the ethnic Vietnamese population lived in thin strips along the coast and (IIRC) one major river, most of Tonkin by land area was non-Vietnamese and mountainous. Honestly, China might find it slightly cheaper to hold on to Annam than to Tonkin.
Given two committed emperors, assuming they aren't terribly short-lived, the three Vietnamese states could probably be incorporated. Within so short a time, though, upland Tonkin would almost certainly be a perennial problem rarely not in some form of revolt. Which isn't so bad - "a perennial problem rarely not in some form of revolt" is a fair descriptor of most Qing provinces in this period.
