The source for this name of this valley, which is the only one in Wales named after Vortigern, is not known. Vortigern's legends do take him to north Wales (especially Snowdonia), but never to the Lleyn peninsula itself. But maybe the legend about his death did travel to this area, as there are many more places that are connected with Vortigern in the direct neighborhood.
At least until around the year 1700 a stone grave covered with a turf mound existed there, which was called Vortigern's Grave (Bedd Gwrtheyrn) by the local population. George Borrow, writing in 1862 (Wild Wales), described it as follows: "It was in a wind-beaten valley of Snowdon, near the sea, that his dead body decked in green armour had a mound of earth and stones raised over it".