There is a much BETTER way as the sword cuts both way.
China reannexed VIETNAM as her former territory of ANNAM just before the French Colonialist stole it from China. Since the Vietnamese got it INDEPENDENCE from the THIEF herself, China action is fully justifiable.
As for Japan, perform what the YUAN DYNASTY emperor did not completed. Annexed Japan and incorporate it inside the Federation of China. Now PEACE in the Far East and South is ASSURED.
Japan will not longer have to feel threatened about energy being cut off because of a S.C.S. blockage by China Navy.
If we read ancient history, when China is STRONG, the region is PROSPEROUS and PEACEFUL.
If the Khmer Krom descendants tried any thing funny like burn Chinese ( meaning the 56 ethnic nationals) just sendin the terminators like what they are doing in Xinjiang and treat them like TERRORISTS.