Vietnamese imperialism and the Second Indochinese War



So I've been studying Southeast Asian politics since this year and I've been talking to many fellow anons on /int/ and researched plenty of things through sites such as Radio Free Asia. Two days after the anniversary of 1973 Paris Peace Accords which drew the Second Indochinese War and the subsequent communist invasion of all three former colonies of French Indochinese (Cambodia is no longer communist today), there's one thing that certainly bothered me: what's with Vietnam and its imperialist stance when it comes to controlling Cambodia and even more so with Laos? The Indochinese Federation proposal, the Japanese occupation of these countries, the rise of communism in Southeast Asia... that says it all.

Is there any reason as to why Vietnam exploits its western neighbors in every way even before the French colonization came? What about the bombing campaigns and how hostile are the U.S. relations with the three countries?