A fun (aborted) timeline on Koxinga's fleet setting up in San Francisco Bay.
The prospects of a Ming exile empire in mainland Southeast Asia are interesting-- OTL saw large numbers of Chinese citizens moving south on individual initiative, and after the Chinese Civil War some renegade KMT set up bases in upland SEA (and some became involved in opiate trafficking not long after). Anyways, imagine-- instead of being destroyed piecemeal like other movements, a Ming restorationist faction relocates to Vietnam's Cao Bang province, taking up the prerogative of "protection" the Ming claimed over the local Mac family (which had seen its attempt at paramountcy over all Vietnam foiled by the Trinh and Nguyen families). Then, maybe some sustained pressure from the Viets or Qing (or pressure from within, as peasants tire of feeding the Ming exiles and force the Mac to act) forces them to flee south in disgrace... but then, miracle of miracles, they receive submission (at least de facto; de jure can come later) from the Khmer principality on the condition that they help hold off Ayutthaya and the creeping Nguyen-led "Southern March" of Vietnam...