There was (and still is) a well founded fear in USforeign policy circles that China is attempting to become an Asian hegemon, and a belief that it is in US interests to prevent this.

The us got involved in the Vietnam war mainly because they thought the Vietnamese communists were going to be proxies other communist governments, especially China.

However, as McNamara and others have said, this fear turned out to be unfounded. Ho Chi Min and his crew were leftists, in that they wanted to radically reform the economy to help poor people, but they were also staunch Vietnamese nationalists who cut their teeth getting Vietnam independence from France. They weren’t trying to replace French colonial domination with Russian or Chinese domination.

In the lead up to the US/Vietnam war, US officials didn’t recognize this, and believed Marxist propaganda that every country w a red flag was going to be allied.

However, relations between China and Vietnam were never actually good and in 1979 just four years after we left vietnam, China and Vietnam had a low intensity border war that lasted until 1990 and saw thousands killed. The war ended, but China and Vietnam still he each other as rivals.

The US still wants to contain China, and largely does this by supporting Asian countries that are resisting Chinese domination. Japan and South Korea have always been a big part of this, but by the ‘90s Vietnam was increasingly cooperating w America to resist China. Vietnam for their part is more than happy to get US support in this endeavor.

TLDR

neither the US or Vietnam like Chinese expansion, so we cooperate to contain them.