He was a younger son of Crofton Vandeleur, member of the Irish House of Commons for Ennis, and his wife Alice Burton, daughter of Thomas Burton of Buncraggy and his wife Dorothy Forster (youngest daughter of Chief Justice John Forster).
[1] John Ormsby Vandeleur MP, who built Kilrush House in 1808, was his elder brother.
He was educated at Trinity College Dublin and entered Lincoln's Inn in 1785.
[2] The law was then a profession which was commonly pursued by younger sons of landed families: Thomas may also have been influenced by the fact that his mother's grandfather John Forster (1668-1720) was an eminent judge who held office as Chief Justice of the Irish Common Pleas.
[3] However, the Vandeleur family subsequently became very unpopular, due to their notoriously harsh treatment of their tenants during the Great Irish Famine of the 1840s, and during the agrarian disturbances later in the century.