The flag of Vietnam, Quốc kỳ Việt Nam, is a yellow five-pointed star on a red field, and it is about as spare as a national flag gets.
From resistance banner to national flag
The Democratic Republic of Vietnam adopted the flag on November 30, 1955. Saigon fell on April 30, 1975, which ended the war, but the two halves of the country were not formally joined until July 2, 1976, when the Socialist Republic of Vietnam was proclaimed and the northern flag became the flag of the whole country.
Earlier flags in the region looked nothing like it. The Nguyễn dynasty used a yellow flag with a blue dragon. South Vietnam flew three horizontal red stripes on a yellow field, a design chosen to signal its anti-communist position.
The current flag emerged in the middle of the 20th century out of the independence struggle against French rule, and its vocabulary is communist and nationalist in equal measure.
Design and symbolism
A red field with a large yellow five-pointed star in the center. Red is revolution, struggle and social solidarity, and it carries the sacrifices made by the population. The star's five points are the five main classes: workers, farmers, soldiers, intellectuals and youth.
Red fields and yellow stars were common across socialist and communist flags, and Vietnam's version fits that family while keeping to a single star.
Where it is flown
The flag hangs at government buildings and schools and is flown on national holidays, with protocols governing its display and hoisting at official ceremonies. Military units and particular government branches use variants carrying their own insignia. Misuse of the flag is regulated by law.
Similar flags
The design is close to the earlier flag of the Viet Minh, the movement central to Vietnam's independence. Soviet and Chinese symbolism shaped it too, as it did most socialist flags of the period, though the single yellow star sets Vietnam's apart.
Terminology
Quốc kỳ is Vietnamese for "national flag." Cờ đỏ sao vàng, the everyday name for this one, means "red flag with a yellow star."
References
[1] Official website of the Socialist Republic of Vietnam (http://www.chinhphu.vn)
[2] The International Federation of Vexillological Associations (https://www.fiav.org)
[3] Historical archives on Vietnamese independence and national symbols (http://vietnamarchieves.gov.vn)
[4] Scholarly articles on Southeast Asian political history