Since there was no declaration of the war, exact dates are sketchy. However, it’s now widely accepted that the Vietnam War started on 1 November 1955 and lasted until 30 April 1975, which is roughly 20 years, or 19 years,180 days to be precise. The first American involvement was in the early 1950s while its direct involvement is said to be from 1963 to 1975. The Vietnam War was the longest war the America was involved in its entire history.
For the Vietnamese, they experienced the First Indochina War (also known as the Indochinese War or Anti-French War) from December 1946 until 1 August 1954 before the Vietnam War (Second Indochina War). So it was nearly 30 years altogether.