Hi Roxana,
The problem, of course, is that Gildas doesn't necessarily name Vortigern - I realise that it's in some manuscripts (from the 12th and 13th centuries) but most commonly there's just the reference to "superbo tyranno." The earliest reference we have is Bede, who maybe used the name from an early version of Gildas but maybe not. Tellingly even Vortigern is not a high king - he's the leader of a council of magistrates. Whether Jordanes' account of Riothamus contributed to the Arthurian legend, or merely confirms the setting, I'm less sure of - this is the huge problem with sub-Roman history. We have these fantastically detailed glimpses into incredibly narrow windows of time and then nothing. What we lack is the continuing and developing narrative - in terms of Arthur we have Gildas, who mentions Badon and then, 300 years later, we have Nennius. Anything else is reconstruction from assumed early narratives in Welsh and Breton folk stories.
Gildas may castigate the tyrants for their lack of historical understanding (of which Gildas had none - just see his descriptions of the Roman period!) but the period he's describing from AA onwards isn't history - it's within a generation and still within the time frame of good oral transmission. He doesn't say - "be like Arthur". He says that the last Roman (and last great leader) was Ambrosius Aurelianus.
The more I study (and it's been over 21 years since I was at university and started looking at this stuff) the more convinced I am that there was never a war leader called Arthur and that he has subsumed Ambrosius Aurelianus as the victor at Badon. Bizarrely, though, I've become more convinced that Vortigern was a real figure.
Pete
God is our guide! from field, from wave, From plough, from anvil, and from loom; We come, our country's rights to save, And speak a tyrant faction's doom: We raise the watch-word liberty; We will, we will,we will be free!