NO. "Ally" is a strong term and usually refers to a country with which the US has a military exchange and/or a convergence on particular political goals that are distinct from the binary relationship between the two states. The United States and Vietnam are on good terms, but since they do not have military cooperation and have minimal political goals outside of their binary relationship, it would be improper to call Vietnam a US ally.
However, Vietnam-US relations are relatively recent. A rapprochement between the Vietnamese government and the US government began in the mid-1990s, two decades after Vietnam defeated the United States in the Second Indochinese War (called the Vietnam War in Western countries and the American War in Vietnam).
South Vietnam was considered an ally of the US.
USA lost to north Vietnam not south. The south was their ally.
Thailand was an ally of the US.
France
Australians.
US, S. Korea, S. Vietnam, Philippines, Thailand, New Zealand.
The US was an ally of South Vietnam; it was up to South Vietnam to repel the invasion from the north, with massive military assistance from the US (which, of course, ultimately failed).
Ally refers to wars; the US is not at war with any nation. The US is friendly with nations and participates in free trade with many countries, Vietnam is one of them.
Australia was an ally of the US and sent troops to fight in both WWII and Vietnam.
US war in RVN commenced in 1955. US war against North Vietnam commenced in 1964.
Yes
Australia, New Zealand, the Phillipenes, South Korea and South Vienam.