Sources conflict on who was Roberts wife. The Complete Peerage says it was
Alice de Sanford who was the mother of Joan. That is a reliable source.
Robert de Vere, 5th Earl of Oxford and 6th Great Chamberlain, having arrayed himself under the banner of Montfort, Earl of Leicester, was amongst those who were surprised with young Hugh de Montfort at Kenilworth a few days before the battle of Evesham and taken prisoner, but he made his peace soon after under the "Dictum of Kenilworth," and we find him employed by King Edward I against the Welsh in the 14th of that monarch's reign [1286]. His lordship m. Alice, dau. and heiress of Gilbert de Saundford, chamberlain in fee to Eleanor, Queen of Henry III, and had, with other issue, Robert, Alphonsus, Hugh, Lord Vere, Joan, and Lora. The earl d. in 1296, and was s. by hiis eldest son, Robert de Vere, 6th Earl of Oxford and 7th Great Chamberlain. [Sir Bernard Burke, Dormant, Abeyant, Forfeited, and Extinct Peerages, Burke's Peerage, London, 1883, p. 550, Vere, Earls of Oxford, &c.]