I think this would require screwing Vietnam and keeping Saigon and its environs.
It would probably also require screwing Thailand. Maybe Thailand remains under Burmese rule for long time, and Burma adopts strict isolationist policies. This leaves Cambodia, including Saigon, as the only viable destination for East Asian and European traders in Southeast Asia.
Greater European influence slowly leads the kingdom out of isolation and European technology and ideas slowly filter in, allowing the kingdom to modernize.