[throwawa5]

> What does Vietnam do as a part of this alliance?

Buys American arms, lets US naval assets use its ports (including aircraft carriers), probably a lot of intelligence cooperation we're not aware of.

> You also better believe that when tensions flare up between Vietnam and China again, Vietnam will be begging the US for help.

The more likely scenario is that the US comes begging Vietnam for help in some future conflict because China shut down the South China Sea as part of a blockade of Taiwan, and used missiles to render all US airbases in the region unusable.