The diocese corresponds to the French civil department of Morbihan, and has been a suffragan to the Archdiocese of Rennes, Dol, and Saint-Malo since 3 January 1859.

From the mid-15th century, the cathedral of Saint Peter was widely famed as the resting place of the remains of S. Vincent Ferrer, O.P.

Fra Vincent had spent the last two years of his life, from 1417 to 1419, in Brittany, a good part of that time in Vannes.

[8] Cardinal Alain de Coëtivy, himself a Breton, was sent by Pope Calixtus as legate to France, and on 2–4 June 1456, he presided over the inauguration of the cult of Saint Vincent in the cathedral of Vannes.

[14] The liturgical life of the cathedral was enhanced, during the Counter-Reformation, and under the influence of the Catholic League, by numerous actions and gifts which added to the solemnity and the richness of various festivals.

In 1702, the bishop installed the Lazarists founded 1625 by Vincent de Paul, as faculty, positions they continued to fill until 1833.

[17] One of the first acts of the French Revolution was the abolition of feudalism and its institutions, including estates, provinces, duchies, baillies, and other obsolete organs of government.

On 13 February 1790. it issued a decree which stated that the government would no longer recognize solemn religious vows taken by either men or women.

Members of either sex were free to leave their monasteries or convents if they wished, and could claim an appropriate pension by applying to the local municipal authority.

[18] These decrees applied to the five monasteries of men in the diocese of Vannes (Redon, S. Gildas de Rhuis, Lanvaux Prières).

At the end of May, its work was presented as a draft Civil Constitution of the Clergy, which, after vigorous debate, was approved on 12 July 1790.

When Bishop Amelot refused to take the oath in February 1791, he was deposed and there was a popular uprising, which was put down by the new departmental authorities.

[28] The pope then recreated the French ecclesiastical order, in the bull "Qui Christi Domini," respecting in most ways the changes introduced during the Revolution, including the reduction in the number of archdioceses and dioceses.

[31] Alexandre Dumas, père, in The Vicomte of Bragelonne: Ten Years Later, the last book of his d'Artagnan Romances.

[32] makes the former musketeer Aramis the bishop of the Diocese of Vannes, on the nomination of Nicolas Fouquet, the Superintendent of Finances of King Louis XIV from 1653 to 1661.