Robert Vermaat's excellent Vortigern Studies site can be found at:
One of the more popular theories is that Vortigern or Wortigernos was a title of rank similar to the Irish ard-ri 'high-king' and not a name at all, or that superbus tyrannus was Gildas' Latin translation of this elusive title. Late Brittonic Uortigern(us) means "over lord". Uor- (Welsh G[w]or, Irish For-) = Common Celtic Uer- / Uor "over/super" and comes from Proto Indo European *UPER "over/super". Tigern = Common Celtic Tigernos "lord" and comes from Proto Indo European *(S)TEG- "covering/house" or *(S)TEIG- "stick/prick". Irish Tigernach comes from Common Celtic Tigernacus and means "of the lord," "lordly," or "lord's possession." It shares a common root with Uortigern, but is not the same name.
Welsh sources added an epithet (Gwrtheyrn Gwrteneu - Vortigern the Thin) which makes it clear they treated it as a personal name.
