possibly the Ventrue Antediluvian
Self (or Inconnu)
The oldest texts mentioning the name cites Veddartha as the sire of Mithras, the 4th generation Prince of London. Later sources also mention Veddartha as sire of Maltheas, a 5th generation Monitor of the Inconnu.
However, according to the accounts of Clanbook: Ventrue it was believed that, by the time of Mitras' Embrace, the Ventrue Antediluvian had already been slain outside of the walls of the Second City. So either Veddartha was Ventrue himself - not destroyed by Brujah or Set, as rumor indicated - or Veddartha was the methuselah responsible for Ventru's Diablerie.
However, Transylvania Chronicles II: Son of the Dragon gave some advantage to the theory that Veddartha never was the Ventrue Antediluvian, when the book gave his name to Maltheas' sire - Maltheas being a 5th generation vampire Embraced many centuries after Mithras. Time of Thin Blood gave us a little more information about Veddartha, who was mentioned in a letter from the 5th generation Inconnu Gracchus to his dearest Hermia, where he voices the opinion of Veddartha on the subject of the Thin-Blooded.
Said letter states that the vampire Veddartha regarded the Thin Blooded as nothing more than appetizers for the 'Feast of the Ages', when the weaker generations would slake and slow the thirst of the Ancients with little cost to the Inconnu's reserves of strength and wisdom (from which he was a part of).
Therefore, a common theory assumes that Mithras was originally a 5th generation who commited Diablerie on another Cainite of more powerful blood. Thus his sire Veddartha would be a 4th generation who's still active during the Final Nights.