Mainland SE Asia has been pushing the chinee back for centuries.
It's all a matter of the cost/benefit analysis.
Did anything that SE Asia had for all those centuries exceed the cost of what it would take for the Chinese to seize and control them, and how much benefit would the Chinese get from it?
If you're talking offshore oil, it is hard for the small countries to defend and easy for China to seize and control, and China needs oil.