The centre of Vefsn is the town of Mosjøen, situated innermost in Vefsnfjord. Outside Mosjøen, the large municipality of Vefsn is dominated by spruce forests, mountains, lakes, agriculture and the Vefsna river. The municipality is served by Mosjøen Airport, Kjærstad
Vefsn was established as a municipality January 1, 1838 (see formannskapsdistrikt). Four other municipalities were later separated from it: Hattfjelldal (1862), the town of Mosjøen (1876), Drevja (1927) and Grane (1927). Mosjøen and Drevja were, however, merged with Vefsn again on January 1, 1962. Elsfjord was merged with Vefsn the same date. The mainland areas of Alstahaug were transferred to Vefsn January 1, 1995.
The municipality is named after the river Vefsna. The meaning of the rivername is unknown.