All that is recorded for Vortimer comes from the Historia Brittonum Chapters 43 and 44.
At length Vortimer, the son of Vortigern, valiantly fought against Hengist, Horsa, and his people; drove them to the isle of Thanet, and thrice enclosed them within it, and beset them on the western side. The Saxons now despatched deputies to Germany to solicit large reinforcements, and an additional number of ships: having obtained these, they fought against the kings and princes of Britain, and sometimes extended their boundaries by victory, and sometimes were conquered and driven back.
Four times did Vortimer valorously encounter the enemy; the first has been mentioned, the second was upon the river Darent (1), the third at the ford, in their language called Epsford (2), though in ours Set thirgabail, there Horsa fell, and Catigern, the son of Vortigern; the fourth battle he fought, was near the stone on the shore of the Gallic sea (3), where the Saxons being defeated, fled to their ships.
After a short interval Vortimer died; before his decease, anxious for the future prosperity of his country, he charged his friends to inter his body at the entrance of the Saxon port, viz. Upon the rock where the Saxons first landed; "for though," said he, "they may inhabit other parts of Britain, yet if you follow my commands, they will never remain in this island." They imprudently disobeyed this last injunction, and neglected to bury him where he had appointed. (4)
One does not necessarily have to accept this as the verbatim truth to realise that it probably preserves some distant folk memory of the initial battles between the Romano-Britons and the Anglo-Saxons. Hengist and Horsa are themselves almost undoubtedly fictitious. There is more reason to suppose that Vortimer may well have existed, but not much.
(1) The river Darent is in Kent, south east of what is now England
(2) Aylesford or Epsford is again in Kent, a town which commands a crossing of the river Medway
The Anglo-Saxon Chronicle for the year 455 records that
This year Hengist and Horsa fought with Wurtgern {that is Vortimer} the
king on the spot that is called Aylesford. His brother Horsa
being there slain, Hengist afterwards took to the kingdom with
his son Esc.
(3) Better known these days as the English Channel.
(4) A different version of this tale is features as one of The Three Concealments and The Three Disclosures of the Island of Britain.