The municipality of Vemundvik was established in 1838 as Namsos landdistrikt/Namsos herred. In 1846 its size decreased as Namsos was separated from it and established as an urban municipality. It had 1,499 inhabitants before the split, and 908 after. On 1 January 1891 another municipality was separated from it; Klinga. Namsos herred (Vemundvik) had 2,475 residents before the split, and 1,088 after.
Areas of Vemundvik lying adjacent to the town of Namsos were later incorporated within the city limits on numerous occasions. On 1 January 1882, an area with 109 inhabitants was moved to the city; on 1 July 1921 an area with 927 inhabitants; and on 1 July 1957, another area with a population of 6. On 1 January 1964, Vemundvik municipality ceased to exist as it was wholly incorporated into Namsos together with Klinga as well as smaller parts of Otterøy and Fosnes. Prior to the merger, Vemundvik had 2,040 residents.