The US somehow mistranslated Vietnamese and understood no to mean yes. From Gareth Porter at thegrayzone.com:
After convincing itself Vietnam would grant it access for missile bases against China, the Pentagon got a hard dose of reality.
When the Pentagon began gearing up for a future war with China in 2018, Defense Department officials quickly realized that they needed access to Vietnamese territory for troops armed with missiles to hit Chinese ships in a US-China conflict. So they initiated an aggressive campaign to lobby the Vietnamese government, and even Communist Party officials, in the hope that they would eventually support an agreement to provide them the permission.
But a Grayzone investigation of the Pentagon’s lobbying push in Vietnam shows what a delusional exercise it was from its inception. In a fit of self-deception that highlighted the desperation behind the bid, the US military ignored abundant evidence that Vietnam had no intention of giving up its longstanding, firmly grounded policy of equidistance between the United States and China.
Vietnam as a key base in US war strategy
Between 2010 and 2017, China developed intermediate-range missiles capable of hitting American bases in Japan and South Korea. To counter that threat, the Pentagon and military services began working on a new strategy in which US Marines, accompanied by an array of missiles, would spread out over a network of small, rudimentary bases and move continuously from one base to another.