Indo - china pennisula is native land of Mon/Khmer/Muon (Viets) people.
Champs came later from Islands in south, they invaded in to our lands and attacked on us. King of Champs was killed when he invaded in to Hanoi. There is failure of them, until we kicked Champs back to Islands in Malaysia and Java.
Who is "us"? Cambodians never considered you to be the same nation. So Vietnam should be part of Cambodia, got it. After you get nuked a few times, Cambodia can reclaim ancient Mon-Khmer Empire later.