I wouldn't seriously expect tales of that time to be much more than legend perhaps based on a kernel of truth, but a fun story is a fun story, and it would be interesting to learn that some Elven chief invited humans into Elvenhan to help only have them turn on her/him and the Elves in general. Is there a reason given for Vortigern's pride, or is it just a name?
(I suspect that the reason the Dalish have Celtic accents is precisely because of those similarities.)
There's a lot of dispute about that.
First of all, there's dispute about whether Vortigern ever existed. A lot of scholars think he didn't. Some think that somebody like him might have existed. Virtually nobody credible believes the classic primary-school version of the myth.
Then there's the issue of what to believe about the 'historical' Vortigern, if there was one. Some historians make him into a Romano-British community leader or warlord unattested elsewhere. Others have connected him with other provably historical figures, like the late Roman would-be emperor Magnus Maximus, or similarly-named individuals that appear on grave markers (e.g. the Ogham marker to "Vortipor").
You can see how differing opinions of what Vortigern might actually have been would change historians' view of why he was called a proud tyrant.
Halsall, who advanced the Maximus theory, suggested (tongue-in-cheek) that anybody who called himself "Great the Greatest" and made a play for the imperial throne would be in for denunciation as a proud tyrant by
somebody; he also thought that the Vortigern figure was hopelessly mythological at worst and unprovably primeval at best, that he originated as a legendary founder of the kingdom of Powys, and that he probably acquired the originally-unconnected story of the
superbo tyranno - Maximus - relatively late in the medieval era.
Others, who modify more traditional readings, suggest that the proud tyrant appellation has more to do with some sort of hypothetical effort to use the Saxons to gain control of Britain for himself, a plan that supposedly backfired.
And then there's the most traditional reading, which in my mind is extremely bizarre, in that Gildas was supposedly making a pun on
Vortigern's name. Never mind that Gildas refers to the same tyrant with three epithets ("proud tyrant", "unlucky tyrant", and "bloody tyrant")...