Douglas Richardson posted this to the GEN-MEDIEVAL board in August 10, 2011: "Complete Peerage 10 (1945): 216-218 (sub Oxford) has a good account of Sir Robert de Vere, 5th Earl of Oxford, who died in 1296. Regarding his death, the following information is provided:

'Robert died before 7 Sept. 1296, and was buried at Earls Colne, his heart being buried in the Grey Friars, Ipswich.'

The footnotes gives the source for this date: Cal. Fine Rolls, i pg. 377. The corresponding item in the Fine Rolls reads as follows:

'[1296]. Sept. 7. Berwick upon Tweed. Order to the escheater beyond Trent to take into the king's hand the lands late of Robert de Veer, earl of Oxford (Exon'), deceased, tenant in chief.'

The Fine Rolls item above may be viewed at the following weblink:

http://contentdm.lib.byu.edu/cdm4/document.php?CISOROOT=/FHMedieval&CISOPTR=35322

From this record, we can be certain that Robert de Vere died shortly before 7 September 1296. In a footnote, Complete Peerage informs us that Robert de Vere died in England, so the king should have been informed of the earl's death soon after his passing.

Recently I was going through the published transcript of St. Edmundsbury Chronicle, 1296-1301, and surprisingly, I located a
specific death record for Sir Robert de Vere, Earl of Oxford. It read as follows:

Source: English Historical Review 58 (1943): 51–78
St. Edmundsbury Chronicle, 1296–1301:

'Dominus Robertus de Wer comes Oxonie migravit ad dominum in crastino
sancti Bartholomei apostolic [25 August].'

Thus it would appear that Sir Robert de Vere, 5th Earl of Oxford, died 25 August 1296. St. Edmundsbury Chronicle assigns no year to the earl's death, but since it only covers events from 1296 forward and since the Fine Rolls pinpoint his death year in 1296, the year indicated is 1296."